FRITS ZERNIKE

1888-1966

Elected For. Mem. R.S. 1956

Frits Zernike was born in Amsterdam on 16 July 1888, both his parents being school-teachers. He was initially trained at Amsterdam University as a chemist and at the age of 24 was awarded a University prize for work on the theory of critical opalescence in gases. Two years later, in 1914, he was responsible jointly with Ornstein for the derivation of the Ornstein-Zernike relation in critical-point theory. His doctorate soon followed, awarded for a theoretical dissertation on critical opalescence and the derivation of Avogadro’s number therefrom.

His first appointment was as assistant at the University of Gronigen to the astronomer Kapteyn but he soon moved over as a junior theoretician to the physics department where in due course he became Professor of Theoretical Physics. Zernike possessed that very rare gift of being at one and the same time a fine theoretician and a first-rate experimentalist. His early appoint­ment at Groningen was at first a little clouded by the fact that his chief, treating him purely as a theoretical physicist, gave him scant laboratory facilities, but a change in Directorate soon rectified this.

Zernike’s experimental work in optics was notable for the elegant sim­plicity of the methods used, which nevertheless led to quite fundamental discoveries. He also invariably brought to bear a profound mathematical insight into all he did. Just as he was in daily life thrifty and economical, so too was he equally economically concise both in the laboratory and in his methods of mathematical analysis. For him it was the neat elegant concise treatment that mattered and he avoided extended computation. He was as equally expert in handling a lathe or milling machine as he was in exploiting theories of statistical mechanics or the subtle refinements of optical coherence theory. He was even known to build a good centrifuge with vacuum-cleaner motors!

In spite of being so sound a theorist and although he commanded quite formidable mathematical techniques, he showed very little interest either in quantum mechanics or in nuclear theory, although there was much activity in both of these fields constantly close to him. First and foremost he was essentially a ‘classical’ physicist, in the best sense of the word, and he remained to the end quite unperturbed by all the ‘modern’ excitements going on around him. To judge from his work and writings his scientific ideal appeared to be Lord Rayleigh and in many respects he resembled him.

393

394Zernike will be remembered chiefly by his two major contributions to

optics, contributions he made from 1933 onward. These two contributions are (a) the creation and application of the principle of phase contrast, and (b) the development of the concept of the complex coefficient of partial coherence in light waves. The first of these has opened up a new and powerful approach in the application of the microscope, especially, but by no means exclusively, to biological and medical sciences. The other virtually began a new branch of theoretical wave optics. It was at a Physical and Medical Congress in Wageningen in 1933 that Zernike first described his phase- contrast technique for the microscope. A year later he extended this to the invention of a phase contrast device which was to be used for testing the figure of concave mirrors. In his papers on these subjects he investigated the diffraction influences on images at focus produced respectively by ( ) variations of amplitude and ( b)variations of phase. Whilst the eye can respond to variations in amplitude (intensity), it does not at all detect phase variations. By a brilliant piece of analysis Zernike proved, most un­expectedly, that invisible effects due to changes and variations in phase could be transformed into visible equivalent changes in amplitude by merely introducing a path change of a quarter of a wavelength into the central diffraction maximum. Thus it was that an invisible phase error or phase alteration in an optical system could be converted into a visible amplitude variation, and thereby be exploited accordingly.

Zernike pointed out that the concept was applicable to the use of the microscope as well as to the figuring of mirrors. Late in the nineteenth century Abbe had already established the fact that fine structure detail could only be resolved by a microscope objective provided that this objective was able to collect the diffracting spectra given by the fine-grain structure of the object. Such an object was conveniently considered to be equivalent to an amplitude grating, consisting essentially of close elements which diffracted different intensity spectra; the finer the structure, the wider the cone angle diffracted and the greater must be the numerical aperture of the objective to collect the diffracted light. Now especially in biological material, there exist numerous quite transparent objects which have fine-grain structural ele­ments of quite transparent material, the structure arising from either thick­ness changes or refractivity alterations. As there is no alteration in intensity across such an object (only in phase) such a phase structure object (as distinct from an amplitude structure object) remains apparently structure­less, when viewed with a microscope critically in focus (some structure does emerge when the object is deliberately set out of focus, but of course detail is then confused). The same situation arises often in metallurgical observation, but here the phase structures emerge as elements of different heights.

What Zernike established, and this was most unexpected, was that no matter how complicated the phase structure, the introduction of a quarter wave retardation (or advance) in the central undiffracted beam from the object, relative to the structure-containing diffracted spectra, completely

Biographical Memoirs

FritsZemike 395transformed the system, the appearance changing from that of an invisible phase object to that of a visible well-contrasted amplitude object. A new contrast was created and the hitherto invisible detail in transparent objects or on metal surfaces appeared now in vivid contrast. By suitable attenuation of beams the method could be made sufficiently highly sensitive to reveal, in contrast, optical path differences of but a few angstroms. This in essence is the basic phase-contrast technique, which has become of such value in biology and medicine. It is true that the partial obscuration of the lens system can lead to the appearance of optical haloes which can be misleading, but Zernike was fully conscious of all of this. Not only did Zernike give the theoretical treatment, he himself constructed the first phase microscopes, no mean feat with the simple means at his disposal.

In 1933, when the writer of this notice was a young research student, working on hyperfinestructure spectroscopy at Imperial College in Fowler’s laboratory, in the adjacent room was Dr C. R. Burch engaged at that time on very difficult problems of perfecting metal mirror reflectors. With explosive enthusiasm Dr Burch at that time demonstrated to the writer an improvement on the Foucault mirror test, with what he called a ‘Zernike phase-plate’. Dr Burch was certainly one of the very first anywhere to recognize the full import of Zernike’s then very new ideas. Indeed none has more authority that he has to comment on Zernike’s invention. I reproduce the following note, sent to me expressly for interpolation in this memoir:

How Zernike brought phase-contrast to England By C. R. Burgh

‘I wrote to Zernike, whom I had not then met, early in 1934 asking his permission to publish the application which I had made of his principle of phase-contrast to the testing of telescope mirrors. “Certainly, publish it” , he replied, “and as I myself have published nothing on it, I will write a paper on the theory, and both papers can come out to­gether.” (He had lectured on it in Holland, and I had heard of it from a Dutch friend.) F. J. M. Stratton communicated the papers to the Monthly Notices of the Royal Astronomical Society: both were read and printed consecutively. He came to London to read his paper, and I recall his first words on entering my laboratory. “Now first I wish to see your apparatus,” and his satisfied comment when I had put it through its paces: “This is the first time I have seen my own test applied to a telescope mirror properly.”

‘Neither of us asked to see the paper of the other, before they were read, and as we found, each had said to himself: “There will be no contradiction.” Nor was there.

‘He brought with him his phase-contrast microscope and showed it to the clinical pathologist to St Bartholomew’s Hospital, Dr R. G. Canti. Canti asked to see blood with it. There was an obvious difference—the

exact nature of which I do not now recall—between his blood and ours. We chaffed him, saying that he had discovered a new disease, and in the classic medical tradition, suffered from it himself. He turned sus­piciously on Zernike: “ It is your thing that is making these cells look different, isn’t it?” “Certainly” , Zernike replied, “but it could not do so unless they were different.”

‘Zernike read a paper on his microscope to the Queckett Micro­scopical Club. In the discussion, one of the oldest members asked that members should not use the new method; it changed the phase of the lateral diffraction spectra and that, he said, was dangerous. Zernike said in reply that on this view, members must immediately give up the microscopist’s standard practice of racking the focus up and down, to see the changes in the appearance of the object, for changing the focus was itself nothing more or less than changing the phases of the lateral diffraction spectra. He would suggest an alternative point of view. “Let us use the method” , he said, “if it is dangerous, we will find ou t!!”

‘We took him to the Strangeways Laboratory, where he demonstrated his microscope on slides from tissue culture material.

‘Later in 1934, I wrote again, asking him to lend his microscope for exhibition at the B.A. meeting of Aberdeen. The first reply came from Mrs Zernike (he was away for a while) “but” , she wrote “I know he will lend it: he is like that!” In due course he wrote: “Play with it as long as you like, and then hand it to So-and-So”—and he named a leading microscope manufacturer, who, however, did not think it worth developing. I collaborated with Canti, photographing fibroblasts in culture, and pathological material. This work ended on his death from pneumonia.

‘(Could the abnormal appearance of his blood, I wonder, have been due to increased lymphocyte cytoplasm refractive index such as was first observed, using phase-contrast, by Keohane and Metcalf, in the lymphocytes of rats which had chronic low-grade virus pneumonia ?)

‘Not until 1942, so far as I know, was any serious observation made with phase-contrast, when J. P. P. Stock—then a general practitioner— observed the adherence of motile organisms to leucocytes, and con­tinuing the observation over many hours, their subsequent gradual loss of contrast, until even under phase-contrast they ceased to be visible —digested extra-cellularly.

‘Then, at the end of the war, Kurt Loos in Austria made his cine film of meiosis in the sperm cells of the grasshopper Psophus , usingthe phase-contrast microscope developed during the war by Zeiss. A. F. Hughes of the Strangeways Laboratory made a film of mitosis in fibroblasts—technically a more difficult achievement—using a phase- contrast attachment which he made himself at Bristol. The American military authority “liberated” the Zeiss phase-contrast objective. English manufacturers woke from their apathy, and rapidly competed

396 Biographical Memoirs

Frits Zemike 397—their product earning the highest praise from Zernike, when he visited us after the war.

‘And then, suddenly, and with something of the enthusiasm said to be characteristic of religious converts, experimental biologists began to say to one another: “ this is what we have been waiting for all these years.” This was just 12 years after Zernike first demonstrated phase- contrast in two laboratories in England. So long may it take for men to perceive the value of what is new.’

Although Zernike is best known for his application of the phase-contrast concept to microscopy, his other contributions to wave optics were of basic importance too. O f especial significance is his work on coherence, about which he considerably extended our understanding. He first drew attention in 1938 to the concept of the degree of coherence of a wave front and indicated that this had many repercussions. For instance, in the classical Young’s slits experiments, everyone knew that the fringes vanish when the slits are broadened. What no one had noticed was that fringes gradually re-appear as the slit broadening continues and it was phenomena like this which Zernike accounted for in terms of degree of coherence. This simple experiment Zernike demonstrated in a lecture in Paris in 1946 and he further extended his ideas when he was invited to give the Thomas Young Oration at the Physical Society of London in 1947. His views on partial coherence not only explained in detail the functioning of the Michelson stellar inter­ferometer, they also added to the theory of microscope image formation. One very satisfying consequence was his demonstration that the two differing theories about microscope imaging, that of Abbe and that of Rayleigh, do not contradict but are simply differences in integration sequence and are therefore actually physically equivalent.

For his studies of the influence of small aberrations on diffraction patterns, Zernike developed special circular polynomials (Zernike’s polynomials) which have proved of value to others in the analysis of large aperture diffraction problems and in the transition from diffraction theory to geometrical optics. Under Zernike’s direction an active group of theoreticians grew up applying such techniques to special problems in optics.

Zernike never abandoned his first love, that of statistical mechanics. In 1939, together with van Lier he wrote a paper on the Senftleben effect, which concerns the small changes in thermal conduction and in viscosity induced in a paramagnetic gas by a magnetic field. From his department there came also papers by students on critical opalescence, specific heats of liquids, molecular order, and phase transitions in binary alloys.

As a man he was witty although at times caustic, a very lucid lecturer, with an admirable command of English. He was slow and hesitant about publishing, shy and retiring, and hating polemic, into which he refused to be drawn. His judgements were considered and never fierce. Once sitting next to him at dinner, the writer tried to draw from him an opinion as to the

relative merits of multiple-beam interferometry and phase-contrast microscopy. Zernike remained quite silent. To precipitate an argument the writer then proposed that with regard to contrast sensitivity (and only this) the multiple-beam method was superior to phase-contrast. There was a long silence then Zernike briefly remarked: ‘It is never superior—at its best it is its equal.’ There was generosity in this remark, for in fact Zernike had never himself used multiple-beam methods, but was quite prepared to accept on trust the formidable sensitivity figure we had quoted to him. One only had to spend a short time with Zernike to appreciate his generosity. He had a sense of fun too and delighted in producing thermodynamics teasers of the perpetual motion variety; just one further aspect of his old love of statistical mechanics.

He had to wait very long for recognition for his revolutionary ideas, sound as they were, were not easy of acceptance when first met. It is on record that when he first offered the design of his phase-contrast microscope to a leading continental microscope manufacturer it was dismissed with the stupid comment: ‘If this was of interest we would have discovered it long ago.’

When at last the value of his work was recognized distinctions followed rapidly. It is to our credit that we in this country were amongst the first both to give Zernike a hearing and then to recognize the importance of his work. In December 1952 our Society awarded him the Rumford Medal. In January 1953 his old University, Amsterdam, conferred on him an honorary degree. In November of the same year the Nobel Prize for Physics was bestowed upon him. He was elected a Foreign Member of our Society in April 1956. He received honorary degrees from the Universities of London, Poitiers and Modena.

Zernike was twice happily married. His first wife, who died in 1943, bore him a son and daughter. In 1954 he married a second time. Some years later signs of Parkinson’s disease made their appearance and this led to a slow but steady deterioration in his health. Although much weakened by his illness he was still able to travel to London to enthral us all by a lecture he gave at a conference on optics. He wrote many papers, being active in publication till 1958. His works are listed below. As a teacher he inspired numerous other publications from his pupils.

In preparing this brief appreciation the writer has been greatly helped with personal notes supplied to him by Professor N. G. van Kampen of Utrecht, a nephew of Zernike. Use has also been made of an article written by Zernike’s pupil, B. R. A. Nijboer, on the occasion of the Nobel Prize award. To Dr C. R. Burch especial thanks are due for his intimate con­tribution regarding the historical reception of the phase-contrast technique. The following list of publications (collected by Dr C. van Winter) has been made available to us through the courtesy of Professor H. J. Groenewold.

398 Biographical Memoirs

S. T olansky

Frits Z 399

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An asterisk denotes a paper read.

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402 Biographical Memoirs

Doctorate theses prepared under guidance of F. Zernike

7 July 1922. H. H. Buzeman.

30 November 1928. G. C. Dibbetz.

21 November 1932 H. C. Huizing.

1 May 1936. H. J . de Boer.8 May 1936. P. F. Lameris.5 July 1938. A. J. Lameris.

23 January 1941. C. J. Bouwkamp.

1 June 1942. B. R. A. Nijboer.7 June 1946. F. W. J. Schweigman.

12 July 1946. R. S. Ketel.

15 June 1948. K. Nienhuis.

26 February 1957. Mej. C. v. Winter. 5 July 1957. M. Bottema.

Optische verschijnselen veroorzaakt door allo- trope moluculen.

Intensiteitsmetingen van de lichtverstrooiing voor enkele organische vloeistoffen.

Absolute metingen der geluidsintensiteit ter bepaling van het minimum audibile.

Over het gladstrijken van krommen.Absorptiemetingen aan enige metalen.Intensiteitsmetingen in het ultraviolette absorp-

tiespectrum van zuurstof.Theoretische en numerieke behandeling van de

buiging door een ronde opening.The diffraction theory of aberrations.Reflectoren. Een onderzoek naar de mogelijkheid

om op een gegeven vlak deel een gevraagd verloop van de verlichting te verkrijgen.

Theorie van het Rayleigh-verstrooiingstriplet van vloeistoffen (afleiding van de formule van Landau en Placzek).

On the influence of diffraction on image forma­tion in the presence of aberrations.

Space-time rotations and isobaric spin.Photometric setting methods in interferometry.