DIATOM TEST SLIDES AND DIATOM TYPE SLIDES

Collectors of antique microscope slides are often interested in Diatom Test slides and Diatom Type Slides. Test slides are slides that contain one or more diatoms typically used for testing the resolution of a microscope objective. Type slides are slides that contain one or more diatoms as specified on the label. Early, diatoms were not the only specimens used as test objects, as noted elsewhere on this site. This page will limit itself to diatoms. Diatom test slides of high quality are still available today from www.diatomshop.com, but they are expensive.

Shown here is a diatom test slide

DIATOM TEST SLIDES:
Early on diatom test slides were simply strew preparations containing predominently one species, but often contaminated by others. They were often labeled Test in addition to the specimen name. These slides are both type and test slides. Overlapping of specimens is common in these preparations as are damaged diatoms and diatom parts; it may take the user some time to find an undamaged single diatom of the type specified on one of these slides. Later in the 19th and early 20th century, test slides usually contained individual examples of 5 to 8 different diatoms arranged neatly in a row, each with a different spacing of their periodic structures striae or puncta. With higher resolution objectives, the diatoms with the more closely spaced features became visible, whereas at lower resolution, the areas appeared uniform. As numerical aperture increases, at first the striae become visible and then, with even higher resolution, the individual puncta become visible. The varieties found on these slides are intended to give samples requiring relatively low numerical aperture for resolution, moderate n.a. and high n.a. In some cases, only special techniques such as extreme oblique illumination, or in modern times, DIC illumination at the highest magnifications will allow resolution of the most difficult test diatoms. Amphipleura pellucida is one of the most difficult to resolve down to puncta, on account of the very close spacing of the puncta which are spaced so near each other that they require resolving power close to the theoretical limit of the light microscope. Resolution in this case requires high refractive index mountant, mounting on the bottom of the coverslip rather than the surface of the slide, and the special techniques noted above. Moller made some slides which he called Prob-Platte which means sample plate, and later he produced aTest-Platte.

Among test diatoms, there are many preparations of a single species, either strewn or individually mounted in collections, including this one. This collection contains the following multi-specimen test slides containing 4-8 specimens:
Klaus Kemp 8 form Test slide mounted in Hyrax,
Klaus Kemp 8 form Test slide mounted in Zrax,
Klaus Kemp 8 form Test slide mounted in Zyrax mounted in the underside of the coverslip,
R.I.Firth 8 test Diatoms in Styrax(1962)
G. Schlueter DT 5 Test-plate
S. Barone 5 diatom Test Slide in commemoration of Edmund J. Spitta(2022)
A earlier 20th C. Moller Testplatte with 5 forms in a microphotographic grid with names and suggested magnifications and numerical apertures(ap) to use with each diatom within that diatom's cell in the grid.

Type slides are more common than test slides. This collection contains type slides in strew form, type slides of carefully placed individual diatoms, and test slides containing several individual types of diatoms. Type slides with accompanying lists on paper include a 100 form slide by Kemp and another by N.B.S. A Moller slide with the 335 diatoms on a photograph grid with names of the diatoms on the grid is also in this collection. Makers of type slides in this collection include W. Firth, R.I. Firth, C.M.Topping, K.D. Kemp, J.D. Moller, Stephan Barone (Diatomshop.com), and Edmund Thum. Among these, the best preparations are by R.I. Firth and S. Barone.

Obtaining Diatom Slides:
In addition to antique slides sold on Ebay, as of 2025, new diatom slides are still being sold by S. Barone at diatomshop.com. High quality slides mounted by the late Klaus D. Kemp were made in the late 20th through the early 20th century and are of good quality; their only negative is that except on special request, diatoms were mounted on the slide with the coverslip mounted on top; he only mounted on the coverslip by special request. S. Barone currently makes very high quality slides, with the diatoms mounted on the underside of the coverslip rather than the slide, using moderately high refractive index mountant, and nicely sealed for preservation. This method, while tedious, and therefore expensive, provides the best resolution of details with the light microscope as used today. Unfortunately slides of some important diatoms, like Amphipleura pellucida, are, as of this writing(2025), NOT available from diatomshop.com.

In the past slides were made with an even higher refractive index mountant, Realgar but this is no longer used as the process is hazardous. Occasionally antique Realgar slides come to market, and these may provide an advantage over the newer slides IF one is lucky enough to have one with the specimens mounted close to, or on the bottom of, the coverslip. It should be noted that the relatively inexpensive C-mount microscope cameras of today do not have the high resolution provided by the film and camera arrangements of the past as used by such people as Spitta, so duplicating some of the best images by Spitta may not be possible with these. Higher resolution microscope cameras are available, but cost several thousand dollars as of the year 2025.