John Wesley Mitchell. 3 December 1913 −− 12 July...

July 2007 12−−John Wesley Mitchell. 3 December 1913

D. J. Barber

published online September 7, 2011Biogr. Mems Fell. R. Soc.

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John Wesley Mitchell3 December 1913 — 12 July 2007

Biogr. Mems Fell. R. Soc.

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John Wesley Mitchell

3 December 1913 — 12 July 2007

elected FRs 1956

By D. J. BarBer

Physics Centre, University of Essex, Colchester, Essex CO4 3SQ, UK

John (‘Jack’) Mitchell was a new Zealander who came to oxford University on a scholar-ship of the commission of the 1851 exhibition. Mitchell’s association with nevill (later sir nevill) Mott FRs during World War ii, when they both worked at the Armament Research Department, afterwards led Mitchell to join Mott at Bristol University, where he began the research into the photographic process for which he is best known. his pursuit of an under-standing of the mechanism through which a latent image forms led to the important discovery of the decoration by silver particles of individual dislocations in silver halide crystals and the mosaic microstructure of them. in turn the decoration technique provided the first clear experimental evidence of the link between plastic deformation and the creation and move-ment of dislocations. in 1960 Mitchell was appointed as professor in the Physics Department at the University of Virginia, where, apart from a brief spell in england as the Director of the national chemical laboratory, he worked happily for some 40 years. During this time his research group published many papers describing and explaining the mechanisms of plastic deformation in metallic alloys, devising and using state-of-the-art methods, thereby adding much to a wider understanding of mechanical properties and strength. All the time, working alone, Mitchell further developed and refined his photoaggregation theory of the photographic process, gaining worldwide recognition and honours for his effort.

Family BackgrounD anD early years in new ZealanD

John Wesley Mitchell (known as Jack to his friends) was born in christchurch, new Zealand, the only child of an American civil engineer who had gone to that country as a surveyor’s assistant. Jack’s father had been born in Derby, connecticut, in 1884, then the fourth male to bear the name John Wesley Mitchell. his forebears, of scottish origins, had emigrated and made lives near the Us eastern seaboard. Jack’s father had married lucy Ruth snowball

http://dx.doi.org/10.1098/rsbm.2011.0007 3 this publication is © 2011 the Royal society



4 Biographical Memoirs

at Waimate, south of timaru, new Zealand, in 1911; Jack was born on 3 December 1913. Jack’s mother, born in 1887, was one of the three children of John and sarah snowball (née Allport) who farmed at inglewood near Mount egmont in the taranaki region on the west coast of north island. John snowball’s family was originally from northumberland, and sarah Allport’s antecedents were members of the Allport and Willett families who had resided in the english counties of Warwickshire and Buckinghamshire and had left home shores to settle in new Zealand about a century earlier.

the Mitchells lived in a small house in the christchurch railway suburb of sydenham, and so Jack’s primary education took place at sydenham school, where he was awarded the Dux Medal before progressing to the christchurch Boys’ high school in 1926 on the strength of a Junior national scholarship. this was followed in 1928 by the granting of the equivalent senior award.

Jack Mitchell’s youthful interests were mostly focused on the wonderful scenery, geology, flora and fauna close to canterbury, where he grew up. his parents encouraged him in a love of nature and in learning about native birds and their songs, wild ferns, plants and trees. he also learnt about the types of lava flows, rocks and minerals to be found in the locality of the lyttelton volcano on Banks Peninsula. As a portent of his exploitation of single crystals in his research career, Jack was especially fascinated by the beautiful lustrous crystallites lining lava cavities, which he later recognized to be varieties of zeolites.

During the early years at secondary school Jack spent many weekends with his father and members of the canterbury Mountaineering club exploring the Banks Peninsula and the peaks in the foothills of the southern Alps. As he matured and, together with club members, became more experienced, the expeditions ranged wider, embracing the southern Alps, Arthur’s Pass and other high passes to Westland. he climbed many high peaks and gained experience of both snowfields and glacier ice. he collected specimens from the different regional zones of metamorphic rocks of the southern Alps, making thin sections of these when he was back in christchurch.

As a 16-year-old Jack travelled by train and then by bicycle through 90 miles of dense rain-forest to the Franz Josef Glacier, where he worked first as a porter and during subsequent sum-mer vacations as an expedition guide. other experiences around this time included working in the upper Rakaia Valley in canterbury as an assistant to c. caldenius of the Geochronological institute of stockholm and accompanying the then Governor General, lord Bledisloe, and his wife on botanical expeditions to collect ferns from rain forest. As a result Mitchell’s first scientific paper (1935) was ‘the vegetation of the Arthur Pass national Park’ (1)*.

Jack Mitchell’s early but lifelong interest in crystalline solids and in the processes of change in the solid state were mostly acquired through a fortuitous informal relationship with Professor R. speight, who had retired from canterbury University college to become the curator of the canterbury Museum. Between 1931 and 1934 Jack accompanied this geolo-gist on field expeditions and was trained by him in crystallography, optical mineralogy and petrology, while formally he was a student reading for a Bsc and taking courses in chemistry, physics and mathematics. Figure 1 shows Jack on one of his many trips.

in 1934 Mitchell was awarded the charles cook Memorial Prize of canterbury University college for his work on metamorphic petrology. he gained an Msc degree with first-class honours in chemistry after a further year of study. his research, which led to an accurate

* numbers in this form refer to the bibliography at the end of the text.



John Wesley Mitchell 5

measurement of the standard potential of zinc and transport properties data for zinc bromide solutions (4), also produced the first single crystal that Mitchell grew, namely zinc.

Armed with an overseas science Research scholarship of the commission for the exhibition of 1851, Mitchell left his much-loved open-air new Zealand life in 1935 and set off for england to take up a fellowship. testimonials written about Mitchell by henry Denham (then head of the chemistry Department at canterbury) and James hight (a prominent politi-cal historian that Mitchell had come to know) described him as ‘probably the most brilliant student in new Zealand today’.

lord Rutherford FRs (PRs 1925–30) and sir henry Denham FRs had been 1851 exhibition scholars before Mitchell, who was destined for oxford University, where Rutherford’s old collaborator, the nobel laureate Frederick soddy FRs, was the Dr lee Professor of chemistry. Mitchell’s eventful journey to england started from Wellington on the Marama, a ship that took him to sydney. Mitchell had been awarded a free passage as part of his scholarship, which enabled him to use the luxurious P&o liner RMs Maloja for his onward voyage. he

Figure 1. Jack Mitchell (with geological hammer) on an expedition into the mountains near canterbury, south island, new Zealand. (courtesy of Roger Kelly; from the library collection of canterbury University college.)




travelled in the company of several young new Zealand scholars including Winston Monk (1912–54), a Rhodes scholar in history, who became a close friend. While awaiting the arrival of the Maloja, their stay in sydney was quite eventful (one of their lodging houses turned out to be a brothel) and they took the chance to see many sights and look up friends. in a move that was to prove influential, Mitchell visited and talked with Professor e. J. hartung, who in the 1920s had studied the photolysis of silver halides with a microbalance at the University of Melbourne. A full and fascinating description of Mitchell’s journey and the early years of his life, both in new Zealand and at oxford, diligently researched by Roger Kelly, are to be found at http://www.kosmoid.net/technology/jackmitchell.

early years in englanD

At oxford, Mitchell joined c. n. (later sir cyril) hinshelwood FRs (PRs 1955–60), working in the Balliol-trinity laboratories and becoming a member of trinity college. his research was concerned with the study of the mechanisms underlying reactions at various pressures between gases that included nitric oxide, hydrogen, deuterium and diethyl ether (see, for example, (2, 3)). he then spent a year tutoring students and in devising a laboratory course in atomic and molecular spectroscopy at the behest of h. W. (later sir harold) thompson (FRs 1946). to meet the course’s needs he learned to make discharge tubes with Pyrex–tungsten glass–metal seals and also a high-intensity sealed and water-cooled helium discharge tube for research on thiophosgene (an interest of thompson’s). this experience informed his later work on high-intensity argon-filled discharge tubes.

Jack Mitchell developed a deep and lasting affection for oxford, not least because his transition from new Zealand was made easier by the understanding and encouragement of hinshelwood. stimulating encounters with Royal society Fellows such as e. J. Bowen, n. V. sidgwick and D. A. Jackson (elected in 1947) of the clarendon laboratory, together with con-tacts with physicists who were arriving in oxford from German-speaking countries all added to his sense of well-being. he took up both squash and cycling. his summer vacations in 1936 and 1937 were spent in France and Germany (where he became proficient in both tongues) and climbing in the Alps. his linguistic abilities later extended to italian, Russian and Japanese.

in 1937 Mitchell became a member of the Faraday society and started to attend its Discussion Meetings. he was uncertain about which research path to follow at this time and the meeting on ‘chemical reactions involving solids’ at the University of Bristol in April 1938 greatly influenced him. Participating in the meeting was the nobel laureate, Robert Pohl, director of the First Physical institute at the University of Göttingen, the leading centre for solid state physics between the two world wars. Pohl presented a paper updating the results of his institute’s research into colour centres, which revived Mitchell’s thoughts about his earlier work on chemical reactions in metamorphic petrology. it spurred him to think that perhaps he should study chemical and photochemical processes in single crystals of alkali and silver halides by means of spectroscopic and electrical conductivity measurements. Physics rather than chemistry seemed to be the best way to further this aim, but at that time employment opportunities in physics were scarce. As so often happens, fate then played a hand: a chance encounter with ernest creswell, educational secretary of the oxford University Appointments committee, an old scholar of and governor of Repton school, led to an opportunity to teach physics at the school, which Mitchell took up in september 1938.




At Repton, where he taught for two academic years, Mitchell’s duties included preparing would-be candidates for university scholarship examinations, which he exploited to study much of the physics that they would learn at university and to set difficult problems to test their suitability. he worked under the wing of A. W. Barton, the head physics teacher, who had started to study radioactive decay at the cavendish laboratory and was continuing to work for a doctorate under the supervision of Rutherford. Mitchell also got involved in school climbing expeditions, and in vacations he went climbing in Wales, scotland, italy and switzerland.

The minisTry oF supply anD ForT halsTeaD

the demands of wartime saw Mitchell join the Armament Research Department of the Ministry of supply at Woolwich Arsenal as a scientific officer in June 1940. An early prob-lem concerned distortions of the brass cartridge cases of the ammunition, when fired, for M2 Browning guns; this problem Mitchell soon attributed to hardness gradients in the cases. these were eliminated by redesigning the dies and changing the annealing regime. Promoted to senior scientific officer, Mitchell was reassigned to work on ammunition for a larger-bore gun and the interaction of armour-piercing shells with targets. it soon became clear that study of the latter required ultra-high-speed photography for which no suitably responsive or intense light sources existed. Mitchell’s solution, born of both previous experience and new research, was a more complex design of discharge tube called an ‘Arditron’ (patented in the UK and UsA by the Ministry of supply (5–8), and used extensively), giving an effective photographic exposure of less than 1.5 μs.

Mitchell was next moved to the Armaments Research establishment at Fort halstead, where he collaborated with two members of the theoretical Physics Division in studies of shockwave interactions. the division was headed by n. F. Mott, and Mitchell attended sem-inars given by Mott on such topics as the physics of metals and of the solid state, including the Gurney–Mott theory of photographic sensitivity. Mitchell was subsequently recruited by Mott to join the illustrious group that he was assembling to work at the h. h. Wills Physical laboratory of the University of Bristol after the war ended. this group included Frank nabarro (FRs 1971) and John eshelby (FRs 1974), Jacques Friedel (ForMemRs 1988) and charles (later sir charles) Frank (FRs 1954). Mitchell accepted the offer of a lectureship in experimental physics from A. M. tyndall FRs and moved to the Royal Fort (h. h. Wills Physics laboratory) in Bristol in september 1945.

universiTy oF BrisTol, 1945–59

Mitchell’s research programme at Bristol started with investigations of the properties of metal surfaces, a subject relevant to Mott’s theory of oxidation, in which the growth of oxide is medi-ated by the tunnelling of electrons from the metal. Mitchell realized that clean metal surfaces were needed for the evaluation of this theory and that this condition would be best approached by the deposition of metals in high vacuum to form thin films on outgassed substrates. Until then, reproducible values of metal work functions had been elusive. A new design of electron gun enabled a hot tungsten emitter and a tungsten collector, separated by a few centimetres, to produce a well-defined beam of 5–10 eV electrons. With this apparatus e. W. J. (Bill; later sir




William) Mitchell (FRs 1986) obtained reproducible values for the work functions of several common metals in the form of polycrystalline thin films on tungsten substrates (9) (Mitchell 1950). limitations of this diode method for surfaces with adsorbed gases turned Mitchell’s attention to the Kelvin method. A design improvement was made by Mitchell’s Australian student, J. c. Rivière, and was used subsequently by Rivière (1957) and several students. A further advance in pursuit of higher vacuum followed when another student, e. B. Dorling, introduced the evaporation of molybdenum to getter residual gases. new approaches, such as self-gettering for the photoelectric method, provided clean stable surfaces for measurements and also greatly advanced high-vacuum techniques. clever in-glass systems were made poss-ible though the outstanding skills of the Bristol Physics Department graduate glassblower, John Burrow. Reproducible values for the work functions and contact potential differences for clean and oxygen-covered metal surfaces were the result, and thus the main aims of the initial programme were met. information about the reactivity of surfaces was lacking, however, so further experiments were designed to address this problem. Among many new results obtained was a confirmation of the theory of cabrera & Mott (1949) regarding the relationship of reac-tion rate to metal film thickness. (When visiting Paris, Mitchell had recruited nicolàs cabrera, son of the exiled spanish physicist Blas cabrera, to join Mott and himself in postdoctorate work. in 1952 cabrera was appointed to the Physics Department of the University of Virginia and a debt was repaid seven years later when Mitchell followed him there.)

Mitchell had harboured doubts about the Gurney–Mott theory (Gurney & Mott 1938) of the photographic process since his first encounter with it at a Mott seminar in 1944. he therefore readily responded to Mott’s suggestion in 1948 that he should start research into photographic sensitivity by using thin-sheet crystals of silver halides, with financial support from Kodak ltd. his keen new interest in the fundamental mechanisms operating in photographic materi-als had major consequences, and it persisted for the rest of his life. the sheet crystals were obtained by passing molten pools of silver halide, held between optically flat glass discs (relics of a Bristol wartime project), through a steep temperature gradient, so that the melt crystallized from one edge (16). typically the resulting disc-like solids consisted of a few crystals. these could be seen in polarized light because they were stress birefringent when still adhering to the plates. single crystals a few millimetres square were cut from the discs and mounted on glass microscope slides with canada balsam. such single crystals, optimally sensitized with silver oxide and then briefly annealed, became the raw material for experiments on photographic sensitivity, in Mitchell’s laboratories and elsewhere.

Mitchell’s student, J. M. hedges, found that exposing the crystals to light produced a developable surface latent image; however, this solarized rapidly. More importantly, an inter-nal latent image was formed that did not suffer solarization and could be developed once the crystal surface layer was removed. the resulting photolytic silver particles were concentrated on boundaries that defined an apparent substructure. longer exposure caused almost continu-ous decoration of linear features within the subgrain boundaries; these were recognized as dislocations (figure 2) (10).

thus the mosaic substructures of crystals (proposed by Darwin (1914) to explain the properties of X-ray reflections from real—that is, imperfect—crystals) and dislocations were first revealed by the print-out effect (10, 11). Until then, growth terraces and etch-pits had been the only visual evidence for the reality of dislocations and had therefore been questionable. now a silver halide crystal that had been exposed after being deformed could be shown to contain arrays of decorated dislocations within its subgrains, thus providing the first experimental evidence linking




dislocations with plastic deformation. Moreover, this provided a transparent model for the dislo-cation processes involved in the plastic deformation of metal crystals with the face-centred cubic structure. small-angle tilt boundaries containing edge dislocations and regular hexagonal twist boundaries consisting of screw dislocations were also revealed (figure 3a), the vector geometry of the latter being analysed by F. c. Frank (Frank 1955). Despite the groundbreaking findings of the hedges and Mitchell papers, there was still some tendency to view ideas about dislocations as ‘Bristol fashion’. this was firmly ended by the observations and recording on cine film of moving dislocations in thin metals foils by transmission electron microscopy performed by the cambridge group under P. B. (later sir Peter) hirsch (FRs 1963).

over the next few years, students in Mitchell’s group, working on both silver halide sys-tems and alkali halides, produced some notable results. the generation of prismatic disloca-tions loops had been predicted by Fred seitz (seitz 1950). together with helical dislocations, loops created as a result of differential contraction were identified, emanating along the twelve ⟨110⟩ directions radiating from small glass spheres that had been embedded in the crystalliz-ing silver halide (21). Prismatic punching in silver chloride crystals containing small amounts of copper chloride was also found by A. Parasnis (figures 3a and 4) (28) and at particles of gold in silver halides by J. t. Bartlett (22).

the decoration of dislocations in sodium chloride was achieved by annealing single crystals in an atmosphere of aurous chloride (15) and the results somewhat matched those obtained using the print-out effect. Dislocation half-loops injected by plastic deformation at the surfaces of the thin-sheet crystals of silver halides, optimally sensitized with silver oxides, could be decorated by particles of photolytic silver (29). expanding the loops into the interior and thus creating interactions with dislocations gliding on intersecting glide planes enabled the determination of glide planes and dislocation Burgers vectors (30, 31). An understanding of the properties of dislocations, subgrain boundaries and various dislocation interactions was gained in this manner. Mitchell’s aim had been to understand the mechanism of latent image formation and thus he stumbled on the hitherto unseen world of dislocations inside crystals;

Figure 2. subgrain boundaries and hexagonal dislocation networks in silver chloride decorated with silver particles by means of the print-out effect (10).(taken from ‘the observation of polyhedral sub-structures in crystals of silver bromide’, by J. M. hedges & J. W. Mitchell, Philosophical Magazine (7) 44, 223–224 (1953). Reprinted by permission of the publisher, taylor & Francis ltd (http://www.tandfonline.com).)




afterwards he said, ‘they were simply there’ (hoddeson et al. (eds) 1992, p. 345). he reviewed the state of the art to about 1960 and related his own story of the discovery and exploration of dislocation phenomena in the book Beginnings of solid state physics (37).

investigations into photographic sensitivity also proceeded. it was demonstrated by t. evans (FRs 1988) that industrial methods of sensitization could be applied to sheet crystals of silver bromide (12, 13), thereby showing that such specimens were ideal for studies of the formation of a surface latent image. Progress included the discovery of new powerful chemical sensitizing agents (see, for example, (14, 18)), proof that the points of emergence of dislocations on crystal surfaces were preferential sites for chemical sensitization (25) and the result that the crystals were sensitized efficiently by a fraction of a monolayer of silver sulphide adsorbed on the surface (26). crystals of the highest achievable purity and perfec-tion were found to be insensitive to latent image formation (17). the range of electrons in such ideal crystals was very small, and chemical sensitization was necessary before there

Figure 3. (a) Dark-field image of a well-defined hexagonal dislocation network (twist sub-boundary) decorated with silver particles in copper-chloride-doped silver chloride. (b) Prismatic and helical dislocations generated by the relaxation of compressive stresses around a 2 μm diameter Hysil glass sphere in the same material. (Taken from ‘some properties of silver chloride containing traces of copper chlorides’, by A. s Parasnis, J. W. Mitchell & h. h. Wills, Philosophical Magazine (8) 4, 171–179 (1959). Reprinted by permission of the publisher, taylor & Francis ltd (http://www.tandfonline.com).)

(a) (b)

Figure 4. Prismatic dislocations extending along ⟨110⟩ directions around a particle of photolytic silver in copper- chloride-doped silver chloride; (a) a [100] direction is normal to the plane of the micrograph, (b) a [111] direction is normal to the plane of the micrograph. (taken from ‘some properties of silver chloride containing traces of copper chlorides’, by A. s Parasnis, J. W. Mitchell & h. h. Wills, Philosophical Magazine (8) 4, 171–179 (1959). Reprinted by permission of the publisher, taylor & Francis ltd (http://www.tandfonline.com).)

(a) (b)




was significant photoconductivity (27). some of these and related findings were at variance with the theories of Gurney & Mott and others, but not all the implications of the results were recognized at the time. however, Mitchell wrote several papers, mostly invited, on the basis of his group’s results and his current thinking about how stable silver nuclei could be formed, thus advancing his photoaggregation theory in the context of the photographic process (for example (19, 20, 23, 24)).

At this point, it seems appropriate to give a more intimate picture of Mitchell’s time in Bristol. of course he took a fair share of the Physics Department teaching load. he enjoyed teaching and he took his duties seriously. he was well known for his clear and well-organized presentations of lecture material but, apparently believing that this was sufficient, he did not readily linger to answer questions from slow-thinking undergraduates at the end of lectures. he ran an intense research group, typically supervising half a dozen students and sometimes having a research fellow or two in addition. A colleague said of him that ‘he works with an unusually high concentration of energy’ and he expected his research students to do likewise. this is a mild description: one of his former students has put it more bluntly, describing him as a slave-driver. Mitchell insisted that his students all take coffee together in the laboratory, rather than go to the Physics Department’s afternoon tea on the third floor of the Physics Department. his reasons are probably revealed by an incident related by Professor P. B. Price of University of california, Berkeley, then a Fulbright scholar. (Price had gone to Bristol to do postdoctoral research with charles Frank. soon after his arrival Price encountered Mitchell, who said that it would be a mistake to work with Frank and that Price should join him—which he did.) Price relates:

While Mitchell was away i observed curious growth twins in zinc dendrites that i grew in his lab. they all showed a feature with a 22° angle. After struggling to understand that angle, i talked with Frank, who suggested that it was the Kronberg angle, which occurs when you lay one lattice on another and rotate it until a number of the lattice points repeat. i told Mitchell about it when he returned, and he was upset. ‘you will never be a good physicist if you ask others for help. Always work out such problems on your own.’

other former students recall that Mitchell rarely gave a face-to-face compliment in recognition of an achievement in the laboratory. More likely he would immediately point out some elusive target that had not yet been reached. J. c. Rivière, in the Mitchell group on a commonwealth studentship from Australia, relates that Mitchell is on record, however, as being very proud of the fact that in his laboratories not only were important basic measurements of work functions being made, but also that Rivière and P. A. schroeder were the first persons in the UK not only to be able to achieve ultra-high-vacuum, but also to measure it. ‘however badly he treated his students within the laboratory, outside it, in the public arena, he was intensely loyal.’

Despite Mitchell’s corralling of his research group, he was outward-looking and made valu-able contributions to the life of the Bristol Physics Department and to physics more generally. one example is his important suggestion concerning the verification of the Aharanov–Bohm effect (Aharonov & Bohm 1959). When R. G. (Bob) chambers proposed to attempt this in an electron microscope, Mitchell suggested that he should use a U-shaped iron whisker to pro-duce the strong static magnetic field necessary. this crucial contribution was duly acknowl-edged in chambers’s first paper (chambers 1960) on the subject.

Mitchell was an intense and private man, but he could also be charming and kind. he occa-sionally entertained his students at his home across the Bristol Downs, where they would be treated to an excellent meal (with new Zealand lamb, of course) and a slide show of beautiful images recorded on Kodachrome during Mitchell’s many trips abroad. on one of these trips, to




new Zealand, Mitchell unexpectedly appeared with a cine camera at the Melbourne wedding of Rivière, who had just recently returned to Australia after gaining his PhD and was marrying an english music student he had met while at Bristol. Mitchell had apparently found out the date and place of the wedding through the Music Department. When he returned to Bristol he summoned the Physics and Music departments and showed a film of the whole wedding, taken some 8000 miles away, in the large lecture theatre of the Physics Department.

the late Professor evans recalled that he joined the research group shortly after Mitchell’s first marriage had ended in divorce (see the later section on homelife, Marriage and Retirement) and when, ‘for six months all he did was work, eat and sleep’, he expected evans to match his commitment. evans related that he won the contest only once—this at the end of a long sunday in the laboratory, where evans had already set the scene by arriving 15 min-utes before the agreed time of 8.30 a.m. it became clear that Mitchell would not leave before evans and so they survived until late in the day on just coffee and scones. eventually Mitchell invited evans to accompany him home, where a good meal was promised. Both were using bicycles as transport and their route was along Whiteladies Road, which reaches the Bristol Downs via the steep Blackboy hill. the established code when cycling with Mitchell was for him to set the pace and, should they need to dismount, for his companion to dismount before Mitchell did. on this occasion evans wickedly engaged a low gear and rode the hill comfort-ably, trailed by a panting, red-faced Mitchell, who would not dismount although his bicycle was equipped with only a single gear. nothing was said when he regained his breath on their way over the Downs.

the tough regime that members of Mitchell’s group experienced seems to have imbued them with a drive to make a mark. the most distinguished was undoubtedly Bill Mitchell (no relation), who had been seconded to Bristol by the Metropolitan–Vickers Research laboratories in Manchester to study for a PhD. his outstanding career included professor-ships at Reading and oxford and the chairmanship of the science and engineering Research council, where his political abilities were clear to see and effectively used. he received a knighthood in 1991. others from Mitchell’s group also became professors (D. J. Barber, e. Braun, t. evans, A. Parasnis and P. B. Price).

Jack Mitchell was himself awarded the c. V. Boys Prize of the institute of Physics in 1955, a prize given for ‘distinguished work in experimental physics which is still in progress’. in the following year he was elected to the Fellowship of the Royal society. By 1959 his research on silver and alkali halides had been widely recognized. But although he was invited to speak at meetings around the world and was made welcome in prestigious research laboratories in Japan and the UsA, he was still a university Reader. Aware also that the bronchitis that he suffered from when living in Bristol would be relieved by a move, he acted on an opportu-nity to join cabrera at the University of Virginia at charlottesville as a professor. there he spent almost all the rest of his life. his appointment, together with those of Doris and heinz Wilsdorf in 1963 and the presence of cabrera, made the university a significant force in the study of crystal defects and plastic deformation mechanisms.

universiTy oF virginia, 1959–63

in the UsA, Mitchell’s attention turned to the deformation behaviour of single crystals of copper alloyed with up to 7.5at.% aluminium. This α-phase system was selected because the




stacking-fault energy was sufficiently low that dislocations would dissociate and thus tend to remain in their glide planes, rendering dislocation interactions more amenable to interpretation than with pure face-centred cubic metals. Much effort was devoted to the preparation of high-quality specimens and methods to be used for studying them, especially surface topography. A technique for growing oriented alloy single crystals with low dislocation densities in vacuum was devised (38). A particular orientation was chosen that was expected to deform relatively simply and thus be suitable for the analysis of dislocation behaviour. such crystal specimens had a [321] axis, uniform square cross-sections and faces of type {1–1–1} and {–14–5}. the faces were rendered optically flat by polishing on chemical-loaded cloth and by electropolishing. Plastically deformed specimens derived from the crystals were studied by several state-of-the-art methods including etch-pitting, interference optical microscopy, slip-line analysis by replica electron microscopy, and direct dislocation imaging by transmission electron micros-copy. Published papers from the group reported mechanisms of slip and hardening processes, together with descriptions of the main types of dislocation interaction that occurred. notable were the absences of repetitive sources of the Frank–Read type (Frank & Read 1950) and internal dislocation sources caused by pile-ups, as theory had predicted. instead, surface sources were found to be responsible for the generation of almost all the dislocations enabling slip. several students obtained their doctorates from these studies and the programme was continued after a short interruption, as described below.

naTional chemical laBoraTory, 1963–64

in the years after he left england, Mitchell was approached many times on the question of his returning. An approach in 1962 by sir harry Melville FRs, the secretary of the Department of scientific and industrial Research, eventually resulted in Mitchell’s being offered and accepting the Directorship of the national chemical laboratory (ncl), a posi-tion that he took up in october 1963. only then, during a discussion with Melville, was it revealed to him that there existed a document, the Report of the Brundrett committee (Brundrett 1964), that proposed, among other ideas, that all the important staff of the ncl’s outstanding and powerful Division of inorganic and Mineral chemistry should be transferred to the Warren spring laboratory. Despite Mitchell’s valiant attempts to prevent this and to save the ncl’s independence, the recommendations of the Brundrett committee were implemented, accompanied by a proposal that the remainder of the ncl be absorbed into the national Physical laboratory and that the post of Director of the ncl be abolished. Mitchell immediately resigned his post and returned to the University of Virginia in the summer of 1964, feeling betrayed and, not least, quietly furious because concomitant with his move to the UK he had sold a charlottesville house that he had loved. the unfortunate circumstances underlying his resignation were recorded in a letter to The Times written by J. s. Anderson FRs (Anderson 1964).

Mitchell had strong views about the importance of a country’s maintaining a sufficient effort in basic research to ensure future innovation and diversification. he predicted danger when the emphasis of R&D was for short-term products and profits. he saw clearly that developed countries had to strive to advance the level of their science and technology if their export capability was to survive. After his experience with the ncl appointment, Mitchell presented an analysis of the organization of basic research in chemistry in Britain in his Jubilee




Memorial lectures of the society of the chemical industry (32), the essence of which was discussed in an editorial in Nature (Anon. 1965), while apparently and significantly being widely noted in Japan.

universiTy oF virginia, 1965–79

Returning to charlottesville as William Barton Rogers Professor of Physics (figure 5), Mitchell resumed his research into the plastic deformation of single crystals of α-phase cop-per–aluminium alloys, while more privately still thinking hard about the mechanisms under-lying the photographic process. single crystals of the alloy system that were oriented with a [321] axis and square cross-sections and polished faces were again produced and deformed. these favoured dislocation glide on a single system, giving a high schmid factor on the pri-mary slip planes (–1–11)[101] and the possibility of etch-pitting the points of emergence of dis-locations on the {1–1–1} external surfaces, thereby with the expectation of facilitating analysis of the slip behaviour. the results indicated the widespread activation of secondary slip, how-ever, causing complicated blocking of the primary slip and resultant hardening. transmission electron microscopy confirmed the etch-pitting observations but showed that the dislocations associated with the secondary systems were generally concentrated in thin regions between the primary slip bands (33). they were created by stresses produced by the superposition on the applied stress of stresses due to packets of primary slip dislocations propagating in opposing directions. Additional deformation experiments with crystals that favoured slip on two equivalent systems, together with calculations (which required the measurement of first-order and third-order elastic constants for different alloy compositions) of the internal stress distributions associated with observed dislocation arrays, led to a new picture of slip initiation and the formation of slip bands (40). According to this view, correlated dislocation creation processes at the opposing surfaces of crystals were initially responsible for the formation of slip bands and then their broadening.

Further well-thought-out deformation experiments on a new single-crystal orientation, subsequent to trying several other orientations, showed that the reproducibility of the pro-cesses responsible for slip band formation was improved and the paths of primary dislocations extended when blocking interactions were avoided. interference microscopy of the surfaces of the test specimens and electron microscopy of shadowed replicas made from them enabled equivalent coplanar slip systems to be distinguished. other tests using the same crystal ori-entation led to several other significant new findings (for example (39)). A crystal orientation was also needed for the simultaneous recording of stress–time and strain–time curves at low temperatures when dislocations of edge character were mainly involved. crystals with a [–12–5] axis proved to be ideal. Detailed analysis of the singly active (–1–11)[101] slip system was aided by confining the deformation to a small ‘waisted’ volume (34).

the many observations resulting from this programme led to a model that attributed the initial formation of slip bands and their subsequent broadening to the correlated cre-ation of dislocations at opposite crystal surfaces. surface sources were normally activated at the yield stress; higher stresses were necessary for the activation of internal sources. confirmation of this model came from experiments in which surface dislocation sources were rendered inoperative by intentionally introducing damage just below the surface. Plastic deformation was delayed until a higher stress was reached. internal sources then




produced dislocations, but without the formation of characteristic slip bands (lovern 1979). some doubt was expressed in the literature about this model that emphasized the role and importance of surface sources over internal sources of the Frank–Read type. But evalua-tion of the resultant dislocation distributions produced by bending tests on crystals with the [–12–5]{1–2–1}{–2–10} orientation gave strong support to the model (36). no evidence for the presence of Frank–Read sources emerged from any of the results of the Mitchell group’s cu–Al alloy programme. A detailed report and discussion of the findings and the resulting model appeared in a 1993 review article (40).

After further work to measure dislocation velocities in the binary α-phase system (34), the programme was extended to binary and tertiary alloys in the α-phase Cu–Al–Ni–Pd system with the aims of studying the effects of composition on yield and flow stresses, of elucidating solute hardening, and establishing how dislocation sources were affected by solute concentrations.

Figure 5. Professor J. W. Mitchell at the University of Virginia in April 1967. (Reproduced courtesy of the special collections, University of Virginia.)




Another interest was the question of how the strength of dislocation-free crystals would compare with the result of theoretical calculation. to test this point, single-crystal filaments of cadmium of constant cross-section, with A-axes and defect-free low-index faces, were grown by distillation in argon. the crystals were strained in tension in a transmission electron microscope and the elastic strain at which they failed was determined from their diffraction patterns. the resolved shear stress on the active glide planes at failure was found to be between 1–11 and 1–15 of the shear modulus, values within the range of estimated theoretical maximum strengths of dis-location-free crystals with atomically smooth surfaces yielding by homogeneous shear (35). the properties of other types of whiskers, including those of non-metals, were also studied.

acTive reTiremenT, 1979–95

on retirement as emeritus Professor, Mitchell was elected a senior Research Fellow, and his thoughts largely returned to and concentrated on the problems of photographic sensitiv-ity. the essential features of the qualitative photoaggregation theory that Mitchell had for-mulated when at Bristol had been rejected by some scientists during the intervening years in favour of the Gurney–Mott theory of photolysis, as modified by hamilton (1977), even though the energy assumed for the liberation of silver and chlorine atoms was in conflict with both experimental results and calculation. in a series of groundbreaking papers, Mitchell set about redefining the photoaggregation theory with calculations of various crucial values and parameters, including the binding energy of Ag+ ions to various silver and mixed silver/gold clusters and the minimum sizes of stable silver clusters in both dry and wet emulsion environ-ments. he then explored the movements and ranges of photoelectrons in silver halide crystals, and the statistics of electron trapping as a function of crystal variables, before introducing the concepts and relevance of donor and acceptor centres to the catalysis of photoaggrega-tion. Mitchell extended these ideas to processes in dye-sensitized silver halide emulsions and indeed for all types of sensitization. Mitchell had also proposed the importance of dislocations to photographic sensitivity in numerous papers, but without attracting as much attention as he felt the concepts deserved. he was pleased to have his ideas justified later when scientists at the Fuji Photo Film company published 1 MeV transmission electron microscopy images of tabular core-shell silver halide crystals cooled to liquid helium temperatures: these revealed dislocations terminating at the crystal surfaces, thus providing sites for enhanced reactivity in chemical sensitization.

Between 1978 and 1995 Mitchell published 33 papers showing that the models he had developed in the photoaggregation theory facilitated a self-consistent discussion of the whole gamut of photographic phenomena from latent image formation to development. During this period he presented his results and ideas at numerous conferences and prestigious industrial venues. he also had appointments as Visiting Professor at the universities of Kyoto, tokyo and Braunschweig. noteworthy are his award of the lieven Gevaert Medal of the society of Photographic scientists and engineers at their 36th Annual Meeting, and his keynote address in 1995 that introduced the symposium ‘Photophysics of silver halides’ in Washington Dc, the content of which is in the symposium proceedings and also elsewhere (41).

so how should Mitchell’s huge effort to produce a comprehensive theory of the photo-graphic process be viewed today? of course, the advances in and rapid adoption of digital image recording have diminished the importance of and commerce in photographic emul-




sions in the past decade and the search has stopped for possible substitutes for the silver halides. in the month of writing this memoir the ending of production of Kodachrome has been announced, the film capable of giving images of outstanding brilliance and hues that inspired many photographers and Mitchell himself. But the initial question still stands and needs consideration now before its significance diminishes further. A definitive answer from experts is not available because the only reviews on the subject are by tani (see, for example, tani 2007), who is in the camp that rejects Mitchell’s theory (the review refer-enced demonstrates its bias by making no mention of any of Mitchell’s numerous papers published after 1981). hamilton, tani and others support a version of Gurney–Mott theory modified to take account of the nucleation and growth concepts of Burton and Berg (see, for example, Berg 1947) and lattice relaxation processes, modelled by computer simula-tions and justified theoretically by hoffman and colleagues (Malik et al. 1999). hamilton & Brady (1959a, b) initially rejected Mitchell’s ideas on the grounds that the properties of silver halide grains in photographic emulsions are very different from those of large crystals. later disagreements turned on the origins and nature of electron trapping and many other mechanistic details. numerous Mitchell papers addressed the differences and criticisms, gradually leading to a sophisticated and very complex description of photographic sensitiv-ity. two schools of thought on photographic sensitivity thus continue to exist: the Mitchell photoaggregation theory and what, for brevity, can be called the direct photolysis theory. the latter is best represented by the hamilton nucleation and growth theory of latent image formation (hamilton 1988) coupled with tani’s electron transfer theory of spectral sensi-tization (tani 1995). it requires dedication to judge which represents the actuality. having read several of the relevant papers, i incline, although i am no expert in this field, to the view that Mitchell’s models, although complex, are logical, self-consistent and probably closest to reality. Whatever the verdict, Mitchell deserves much credit for bringing to light the importance of dislocations as locations of chemical reactivity, with surface and point defects as enabling entities in all solid state processes, not only those photographic. some of the Mitchell groups’ early experimental observations and his later theoretical understanding have played a big part in great technological advances, exemplified in incredibly sensitive silver halide microcrystals as double-structure or triple-structure grains in fast photographic films (such as Fujicolor super G 400).

home liFe, marriage anD reTiremenT

considering the intensity of Mitchell’s approaches to research, teaching, travel, photography and other interests such as climbing, exercise and exploration, it is hardly surprising that his first two marriages ended in divorce. the first, apparently of short duration, has always been and remains a mystery. i have known no one who can shine any light on this union. Mitchell himself seems never to have spoken about it, beyond revealing (to his then student, trevor evans) soon afterwards that it had both occurred and ended. the second marriage, in 1968 to Jo overstreet long, took place at Belleair Beach, Florida, and lasted seven years, resulting in a lasting warm relationship with his stepdaughter, Jody Karen Fidler (née long). his third marriage to Virginia Jacobs hill in 1976 (figure 6) continued until her death. Virginia was the widow of chester James hill Jr, who had been a Professor of Psychology at lawrence University, Appleton, Wisconsin. she understood the demands of teaching and research into




complex problems, giving Mitchell the support and love that enabled his remaining years to be very productive, satisfying and happy. Although nothing is known about Mitchell’s first short marriage, it seems safe to assume that there were no offspring; nor were any children born of the other two marriages.

it was after his retirement that Mitchell worked out many of the quantitative aspects of the photoaggregation theory of latent image formation. Virginia typed all his papers, accompanied him on almost all of his travels and made a welcoming home for the visits of many friends. even when Mitchell lived alone in the early years after he moved to charlottesville he was a gracious and accomplished host. Buford Price (Professor P. B. Price, Berkeley) sometimes stayed with Mitchell when he was recruiting graduates for General electric, schenectady (where Price then worked); he recalls that he was treated to superb meals. he also remembers retiring and waking up to find that Mitchell, having stayed up late to prepare a lecture, had risen early to cook shirred eggs and other delicacies for breakfast. Price also recalls Mitchell’s prodigious memory for detail—when he mentioned to Mitchell that he and his wife would be staying in Kyoto for a while, Mitchell reeled off train departure times for nara and the opening times for several temples.

the Mitchells’ fine garden provided both relaxation and exercise; in springtime it was a riot of flowering bulbs, together with beautiful azaleas and dogwoods, and bird feeders attracted cardinals, chickadees and crested tits. Virginia also allowed Mitchell the freedom to be his obsessive self. An example is his 80th birthday, when many former students and colleagues travelled from afar to celebrate with him at the University of Virginia. he insisted on organ-izing the entire affair himself, also deciding where each of his guests would sit at the formal

Figure 6. Jack and Virginia Mitchell in the garden of their home at Kent Road, charlottesville, in the summer of 1979. (online version in colour.)




dinner and putting out the place cards. he also organized an exhibit of posters and photo-graphs, mounting these on a wall himself.

Mitchell considered himself to be a man of three countries and indeed he was, enjoying the best of each. he never lost his affection for new Zealand and the many aspects of its natural beauty. he visited it 11 times between 1945 and 1990, making four of the trips with his wife after his retirement in 1979.

Mitchell lived at home until his death at the age of 93 years, taking pleasure in reminiscing with his stepdaughter, Jody Fidler, and other visitors about his successes, his many interests and experiences, and his travels. his passing was marked by the Physics Department of the University of Virginia, where Professor Keith Williams presented an appreciation and memoir. Mitchell showed his lasting affection for trinity college, oxford, in a generous legacy to sup-port undergraduates, as acknowledged in the college Annual Report.

acknowleDgemenTs

the writing of this memoir was greatly assisted by a document about his life and achievements written by Jack Mitchell himself. An excellent source about Mitchell’s early life has been the information researched and presented by Roger Kelly, whom i thank very much. this can be found at http://www.kosmoid.net/technology/jackmitchell. i also warmly thank Mrs Jody Fidler for invaluable information. For further assistance i am indebted to the late Professor trevor evans FRs, Professor David Bassett, Professor ernest Braun and Professor Bascom Deaver, Dr chris evans, Professor Arawind Parasnis and Professor P. Buford Price, Dr John Rivière, Professor Keith Williams and, not least, Dr Mike stowell FRs for encouraging me to undertake the task.

reFerences To oTher auThors

Aharonov, y. & Bohm, D. 1959 significance of electromagnetic potentials in the quantum theory. Phys. Rev. 115, 485–491.

Anderson, J. s. 1964 letter to the editor. Behind a resignation. The Times, 1 July.Anon. 1965 editorial. Government policy and industrial science, 1965. nature 207, 113.Berg, W. F. 1947 latent image formation in photographic silver halide gelatine emulsions. Rep. Prog. Phys. 11,

248–297.Brundrett, sir Frederick 1964 Report of the Committee on Technical Assistance for Overseas Geology and Mining

and policy on the recommendations of the committee (presented to Parliament by the secretary for technical cooperation by command of her Majesty, May 1964). london: hMso.

cabrera, n. & Mott, n. F. 1949 theory of the oxidation of metals. Rep. Prog. Phys. 12, 163–192.chambers, R. G. 1960 shift of an electron interference pattern by enclosed magnetic flux. Phys. Rev. Lett. 5, 3–5.Darwin, c. G. 1914 the theory of X-ray reflexion. Phil. Mag. (6) 27, 315–333.Frank, F. c. 1955 hexagonal networks of dislocations. in Defects in crystalline solids (Proceedings of a conference

held at h. h. Wills Physical laboratory, University of Bristol, July 1954) (ed. J. W. Mitchell), pp. 159–168. london: the Physical society.

Frank, F. c. & Read, W. t. 1950 Multiplication processes for slow moving dislocations. Phys. Rev. 79, 722–723.Gurney, R. W. & Mott, n. F. 1938 the theory of the photolysis of silver bromide and the photographic latent image.

Proc. R. Soc. Lond. A 164, 151–167.hamilton, J. F. 1977 in The theory of the photographic process, 4th edn (ed. t. h. James), pp. 105–132. new york:

Macmillan.hamilton, J. F. 1988 the silver-halide photographic process. Adv. Phys. 37, 359–441.hamilton, J. F. & Brady, l. e. 1959a electrical measurements on photographic emulsion grains. 1. Dark conductivity.

J. Appl. Phys. 30, 1893–1901.




hamilton, J. F. & Brady, l. e. 1959b electrical measurements on photographic emulsion grains. 2. Photoelectronic carriers. J. Appl. Phys. 30, 1902–1913.

hoddeson, l., Braun, e., teichmann, J. & Weart, s. (eds) 1992 Out of the crystal maze: chapters from the history of solid state physics. oxford University Press.

Lovern, T. N. 1979 The role of surface and internal sources in the plastic deformation of α-phase copper-aluminium alloys. PhD thesis, University of Virginia.

Malik, A. s., Blair, J. t., Bernett, W. A., Disalvo, F. J. & hoffmann, R. J. 1999 Photographic sensitization of the AgBr(100) surface and the effect of Au and s in latent image formation: a detailed theoretical mechanism. J. Solid State Chem. 146, 516–527.

Mitchell, e. W. J. 1950 some contact potential measurements of metals and semiconductors. PhD thesis, University of Bristol.

Rivière, J. c. 1957 contact potential difference measurements by the Kelvin method. Proc. Phys. Soc. Lond. B 70, 676–686.

seitz, F. 1950 Prismatic dislocations and prismatic punching in crystals. Phys. Rev. 79, 723–724.tani, t. 1995 Photographic sensitivity: theory and mechanisms. new york: oxford University Press.tani, t. 2007 Review of mechanisms of photographic sensitivity. Imaging Sci. J. 55, 65–79.

BiBliography

the following publications are those referred to directly in the text. A full bibliography is available as electronic supplementary material at http://dx.doi.org/10.1098/rsbm.2011.0007 or via http://rsbm.royalsocietypublishing.org.

(1) 1935 the vegetation of the Arthur Pass national Park. in Handbook of Arthur Pass (ed. R. s. odell), pp. 93–98. christchurch, new Zealand: Whitcomb & tombs.

(2) 1937 (With c. n. hinshelwood) the inhibition of photochemical reactions by nitric oxide. Proc. R. Soc. Lond. A 159, 32–45.

(3) (With c. n. hinshelwood) the influence of hydrogen and deuterium on the thermal decomposition of diethyl ether in the low pressure region. Proc. R. Soc. Lond. A 162, 357–366.

(4) 1939 (With h. n. Parton) the activity coefficients and transport numbers of zinc bromide at 25 °c, from eMF measurements. Trans. Faraday Soc. 35, 758–765.

(5) 1946 Luminous discharge tubes. UK Patent no. 574,581, 23 January 1946. Application no. 21822, 29 December 1943.

(6) 1949 Gas-filled discharge tubes as light sources for high-speed photography. Trans. Illum. Eng. Soc. Lond. 14, 91–104.

(7) Light sources for high speed photography (Permanent Records of Research and Development, Monograph no. 4.401(b)). london: Armament Research establishment, Ministry of supply.

(8) 1951 luminous discharge tubes. Us Patent no. 2,567,491, 9 December 1951. Application no. 814,720, 9 June 1945.

(9) (With e. W. J. Mitchell) Work function of Ge. in Semiconducting materials (Proceedings of a con-ference held at the University of Reading, July 1951) (ed. h. K. henisch), pp. 148–150. london: Butterworths.

(10) 1953 (With J. M. hedges) the observation of polyhedral sub-structures in crystals of silver bromide. Phil. Mag. (7) 44, 223–224.

(11) (With J. M. hedges) some experiments on photographic sensitivity. Phil. Mag. (7) 44, 357–388.(12) 1955 (With t. evans & J. M. hedges) A further contribution to the theory of photographic sensitivity. J.

Photogr. Sci. 3, 73–91.(13) 1956 (With P. V. McD. clarke) experiments on photographic sensitivity. J. Photogr. Sci. 4, 1–20.(14) 1956 crystals of silver halides. Address delivered at the award of the 11th charles Vernon Boys Prize. in Year

Book of the Physical Society, London, pp. 34–36. london: Physical society.




(15) 1957 (With D. J. Barber & K. B. harvey) A new method of decorating dislocations in crystals of alkali halides. Phil. Mag. (8) 2, 704–707.

(16) (With n. F. Mott) the nature and formation of the photographic latent image. Phil. Mag. (8) 2, 1149–1170.

(17) on the electronic conductivity of crystals of silver halides. Phil. Mag. (8) 2, 1276–1281.(18) Dislocations in crystals of silver halides. in Dislocations and mechanical properties of crystals

(Proceedings of an international conference, lake Placid, new york, 1956) (ed. J. c. Fisher, W. G. Johnston, R. thomson & t. Vreeland Jr), pp. 69–91. new york: Wiley.

(19) Die photographischen empfindlichkeit. Photogr. Korresp. 1 (suppl.), 1–35.(20) Photographic sensitivity. Rep. Prog. Phys. 20, 433–515.(21) 1958 (With D. A. Jones) observations of helical dislocations in silver chloride. Phil. Mag. (8) 3, 1–7.(22) (With J. t. Bartlett) the decoration of dislocations in crystals of silver chloride with gold. Phil. Mag.

(8) 3, 334–341.(23) the photolysis of crystals of silver halides. in Photographic sensitivity (tokyo symposium, october

1957) (ed. s. Fujisawa), vol. 2, pp. 47–64. tokyo: Maruzen.(24) Photographic sensitivity. J. Photogr. Sci. 6, 57–80.(25) the nature of photographic sensitivity. lecture delivered at the award of the 11th Renwick Memorial

Medal of the Royal Photographic society. J. Photogr. Sci. 5, 49–70.(26) the sensitization of crystals of silver halides with sulfur compounds. in Wissenschaftliche Photographie

(Proceedings of a conference on photographic science, cologne, Germany, 24–27 september 1956), pp. 29–35. Darmstadt: helwich.

(27) 1959 (With e. A. Braun) Photoconductivity in crystals of silver bromide. J. Phys. Chem. Solids 8, 297–300.

(28) (With A. Parasnis) some properties of silver chloride containing traces of copper chlorides. Phil. Mag. (8) 4, 171–179.

(29) 1960 (With J. t. Bartlett) the generation of dislocation loops at the surfaces of crystals of silver bromide. Phil. Mag. (8) 5, 445–450.

(30) (With J. t. Bartlett) Dislocations with Burgers vectors of a⟨100⟩ in crystals of silver bromide. Phil. Mag. (8) 5, 779–802.

(31) 1961 (With J. t. Bartlett) interactions between dislocations with Burgers vectors at 120° in crystals of silver bromide. Phil. Mag. (8) 6, 271–275.

(32) 1965 the organization of basic research for the British chemical industry (Jubilee Memorial lecture of the society of chemical industry). Chem. Ind., 908–935.

(33) 1967 (With J. c. chevrier, B. J. hockey & J. P. Monaghan Jr) the nature and formation of bands of defor-mation in single crystals of α-phase copper-aluminium alloys. Can. J. Phys. 45, 453–479.

(34) 1970 (With R. B. schwarz) Dynamic dislocation phenomena in single crystals of cu–10.5at.%-Al alloys at 4.2 K. Phys. Rev. B 9, 3292–3299.

(35) (With J. c. crump iii) strength of near-perfect single crystals of cadmium. J. Appl. Phys. 41, 717–722.

(36) 1979 (With W. e. nixon & M. h. Massey) Dislocation generation and displacement in single crystals of cu–10.5at.%-Al alloy deformed in bending. Acta Metall. 27, 943–950.

(37) 1980 Dislocations in crystals of silver halides. Proc. R. Soc. Lond. A 371, 149–159. (Also published by the Royal society in The beginnings of solid state physics (ed. sir nevill Mott).)

(38) 1981 (With W. E. Nixon) The yield stress of single crystals of α-phase copper–aluminium alloys. Proc. R. Soc. Lond. A 376, 343–359.

(39) 1989 (With S. K. Ray) The yield stress of single crystals of α-phase Cu–Ni–Pd and Cu–Pd alloys. Proc. R. Soc. Lond. A 423, 267–278.

(40) 1993 Elementary processes in the formation of slip bands in single crystals of α-phase Cu–Al alloys. Phys. Status Solidi A 135, 455–466.

(41) 1995 the basic concepts of the photoaggregation theory. J. Imaging Sci. Technol. 39, 193–204.



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