TEST OBJECTS:

INTRODUCTION:
Since the 1820s microscopists have depended on various objects to test the resolving power and aberrations of their microscope objectives. The objects were usually a specimen that had some periodic or distinct structure such as hairs, insect scales, and later, diatoms. Later on of course, artificial test objects of increasing accuracy and sophistication became available. In using the natural subjects as test objects, it was assumed the distance between the periodic structures was the same in each example of the same subject, but this has proven not precisely to be the case, at least in some. As time went on, diatoms have proven to be the most reliable (and popular) of the natural test objects, though man made test objects and devices such as apertometers, ruled gratings, the star test, aberration test slides, and Abbe's test plate, etc have become more reliable and more frequently used standards.

For low power objectives, larger subjects like the proboscis of the blowfly, were often used. Another good test object is a thin section stained histology slide, such as that of the glomerulus of the human kidney. It is very important to realize that being a test for a certain numerical aperture does not mean this is a simple test. For instance in some cases, it is assumed the user knows that a particular object must be viewed with oblique light and from what direction this is best directed. This is especially true at powers above 20X or n.a. 0.45 or so. In some cases, the object in question must be mounted in an unusually high refractive index mountant for optimal visualization; this is well illustrated by Amphipleura pellucida, where resolving the areolae with a light microscope is likely impossible without both the high RI mountant AND properly oriented oblique illumination. The use of oblique lighting from a particular direction turns out to be true even for the scales of the wings of the Blue butterfly Morpho menalus. This is not true for all cases as the areolae of the Synapta (Ulnaria) capitata diatom is best resolved by darkground illumination, where oblique lighting comes from all directions. This page features some examples of test-objects in my collection along with associated information. It is important to realize that modern mounts of these same objects may differ from the ones originally described, and therefore current conclusions may not be the same as those in years gone by. In some cases, resolution is better than in the past using current equipment, but in some cases this is not true due to factors such as the type of mounting material, thickness of coverslip, thickness of slide and if the subject is mounted on the slide or the bottom of the coverslip. If the user is interested in seriously determining the quality of an objective, the objective tests such as the Abbe test plate, an Apertometer, etc may be best. If interested in exploring the tests like they once were, then the subjective tests will be appropriate, and also entertaining. This page is about the natural test subjects.

HISTORY:
In 1827, in the Quarterly Journal of Science, Literature, and Art, Goring published an article on test objects which included scales of the wing of the butterfly Pontia brassicae, scales of the wings of a common house moth, the scales of the diamond beetle, the point of the leaf of a hedge moss, bat's hair, white numbers engraved on a black background, and the foot of a blue bottle fly. He described the use of these not only using transmitted light, but also as opaque subjects. He emphasized that resolution was far more important than magnification, for magnification without improved resolution resulted in inferior images.

The nonachromatic small Cary-Gould pocket microscope was invented in the 1820's and Gould published a booklet about using his microscope starting in 1823. In 1835 he first added illustrations of test objects (left). As compared to later illustrations using microscopes with higher resolution, these illustrations showed features only discernable with Gould's nonachromatic objectives and diatoms were not included.

Charles Topping was making commercial microscope slides by 1841. In 1854, Hogg, in his first edition of The Microscope: Its History, Construction, and Applications... stated that Mr Topping, the excellent preparer of microscopic objects generally furnishes three kinds of test-objects which he carefully covers with the thinnest glass, in order that object glasses of the highest powers may be used to examine them. In this list, there were 9 slides of scales, 6 of hairs, but only only 3 diatoms listed; he stated that the diatoms were much more difficult to define than the other objects. He went on to quote Dr Goring that the study of the manner in which these objects are exhibited is of great importance...for this reason the Infusoria are considered the most valuable and in illuminating them, oblique diverging rays appear to be essential...the degree of obliquity varying with the different specimens.... Topping continued to sell test slides for many years, dying in 1874. Many of his test slides from my collection, are shown to the left.

Many other slide preparers made and sold test slides. Shown to the left are two slides by Wheeler, the middle one sold by Watson, and the third made by Darleston. Other makers included J.T. Norman, J.D. Moller, and others. Test slides by Norman are shown to the right; the slide with the blue paper cover is actually made of cardboard, and the diatoms are tmounted between two coverslips contained mounted on the top of that slide.

Moller made slides with diatoms mounted on a microphotographic grid starting in the 1870s. One of my slides is a test slide with 5 different diatoms within a photographic grid. Each square of the grid indcates the name of the diatom and the magnification and numerical aperture required to resolve the areolae of that diatom. Another superb test slide by R.I. Firth (1853-1982) with 9 different diatoms, and dated 1962 is also on this site.

Diatom test slides (as well as artistic arrangements of diatoms and butterfly scales), continued to be made by Klaus Kemp from about 1993 through about 2022 when he passed away. High quality diatom test slides are still made by Stefan Barone of diatomshop.com today. Creating slides with micromanipulated diatoms either as test slides or artistic arrangements takes considerable effort and time, therefore these modern slides, just like the antique ones, are expensive.

For those that desire an in depth discussion of the nuances of using the test objects mentioned here, I suggest you read the chapter on Test objects in Spitta, Edmund, Microscopy, The construction, Theory and use of the Microscope. The third edition was published in 1920, and is readily available both on the internet and as hard copies. Some the images in it are perhaps the best ever taken, in particular those of Amphilpleura pellucida.

CLICK ON IMAGES FOR ADDITIONAL AND LARGER IMAGES