Zernike’s colour phase-contrast.

Frithjof A.S. Sterrenburg, Netherlands

Some lore surrounding the history of phase-contrast optics has never been documented as far as I know, but is simply too nice to be lost in the mists of time. Hence this brief contribution, and I can vouch for its authenticity.

As is generally known, the Dutch physicist prof. Frits Zernike invented phase-contrast (for which he received the Nobel prize in 1953) in the 1930s and sold the manufacturing rights for this new technique to Zeiss before the Second World War. However, it seems that Zeiss mainly bought these to keep this new-fangled gadget off the market, because at that time they were smitten with some invention by Siedentopf, I believe, and wanted to concentrate their marketing efforts on that. Thus, large-scale production of PC sets was not started before the war, which explains why the PC images published in the early literature on PC seem to be limited to a few pictures of buccal epithelial cells made by Zernike himself. With the advent of the war, Zeiss had things other than microscopy to keep them busy, of course. After the war, all German patents were declared war booty by the Allies and everybody started making PC optics.

However, Zernike had more up his sleeve. Perhaps partly to take revenge, he next patented achromatic PC, and finally colour PC.

In the achromatic PC objectives, the phase-ring inside the objective is achromatized. That is to say: the l/4 phase-shift in the phase-ring is not limited to green, but is maintained over a wider range of the spectrum. Hence a green filter is not used with these lenses.

In the colour PC objectives, use is made of dispersion of the special phase-ring. A small phase-shift, or none at all, results in a red colour (hence the background is red), a greater phase-shift is represented by a blue colour of the structure. Of course illumination is by white light. The method is not as sensitive to optical path differences as interference contrast, you need fairly large phase-shifts in the object, but for such cases it works fine. The phase ring has a light greyish-tan tinge, the rest of the objective is clear, you use an ordinary phase annulus in the condenser and polarization is not involved. 

For manufacturing of his achromatic and colour PC, Zernike went to the firm of Nedoptifa in Zeist. This was founded by the mathematician (Miss) Dr. C.E. Bleeker, a shining example of women’s emancipation. An important source of income of the firm was lab instruments like resistance standards and optical sound registration equipment for the talkies, but they also produced a range of rather nice microscopes. Their chief designer was Dr. Jaap van Zuylen, who also worked on the question of whether van Leeuwenhoek indeed ground the lenses for his microscopes (he did) and published his findings in 1981. I knew the late Dr. van Zuylen personally and he was the source of my information.

At least in the early years (ca. 1950), Zernike visited Bleeker every now and then, cycling from Groningen to Zeist (100 miles or so) when the weather was nice, taking a small box with phase rings (these were films) with him for the mechanics to produce another batch of PC objectives. I don’t know whether that was true for all Nedoptifa PC objectives, but Bleeker never manufactured large numbers of microscopes; the number of achromatic PC objectives produced was even smaller and that of colour PC objectives was smaller yet.

In the 1970s, I received some Nedoptifa PC objectives from Jaap van Zuylen, including a colour PC objective with a phase ring he told me was definitely made by Zernike himself. It’s a 40/0.65 achromat and the two photomicrographs presented here show what Zernike’s colour PC looks like. The Surirella sp. shows two longitudinal rows of very fine black “dots” that are clearly resolved and are near the theoretical limit of resolution of an NA 0.65 objective. The Nedoptifa PC objectives had large-diameter phase rings, that of the 40/0.65 corresponds to NA 0.42.

The Auliscus sp., with its much more heavily silicified valve, illustrates the range of colour obtainable with this form of colour PC.

Comments to the author Frithjoff Sterrenburg are welcomed.

*****

Above: Diatom, Surirella pandura, colour phase contrast.

Above: Diatom, Auliscus reticulatus, colour phase contrast.

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