About the year 1826, Dr. Goring discovered that the structure of certain bodies could be readily seen by some microscopes, but not by others. He called these bodies as test objects. In the course of his experiments, he was led to the conclusion that there were two distinct powers in a microscope, namely, defining and penetrating, and that an object glass might posses the one almost to perfection, and yet be totally devoid of the other, or might be perfect in both. At present, however, it is the opinion of the most celebrated opticians that the terms of definition and defining power are the only expressions requisite to be employed to denote the good or bad qualities of any microscope. The test objects now generally employed for ascertaining the merits of any achromatic combination may be divided into three kinds, namely, hairs of animals, scales from the wings and bodies of insects, and the siliceous coatings of recent and fossil infusoria, those of the latter kind being the most difficult of all to define. The following list contains a few of the test objects to which allusion has been made.
Animal hairs are used as test objects for microscopy.
Bat. The hair which forms one of the test objects is obtained from the species of bat before mentioned, inhabiting some parts of India. The principal parts of the hair that form a test form a test of the defining power of a half inch object glass, are the delicate points, or scale like projections, that surround the upper edge of each whorl, which should be shown exceedingly sharp. In some of the species of English bats, the whorls are arranged in a spiral form.
Mouse. The hair of this little animal differs materially both in structure and in size from that of the bat. In some parts of the hair the internal structure is cellular, there being three or more cells in each row, the color of the hair depending upon the greater or less amount of the black pigment contained in the cells. When viewed with a power of one or two hundred diameters, all the light parts should be shown distinctly from the dark, and the line of separation of the two correctly defined. When viewed as an opaque object, this hair is very beautiful.
Dermestes. This very remarkable hair is obtained from the larva of a small beetle, commonly met with in bacon, hams, and other dried animal substances. It is covered over with brownish hairs, the longest specimens of which should be selected. When one of these is viewed with a magnifying power of two hundred diameters, the upper part of the shaft, near the head, is provided with several larger and more obtuse spines, forming a knob. This very beautiful hair now forms a good test of the defining power of a half inch object glass. The hairs of the mole, the rabbit, and the squirrel, are also used as test objects.
Scales on the wings or bodies of insects are also use as test objects for microscopy.
Pontia Brassica, the common spring tail. The body and legs of these tiny creatures are covered with scales of great delicacy. The surface of them appears covered with immense numbers of wedge shaped dots, arranged so as to form both longitudinal and transverse wavy markings when magnified one thousand two hundred diameters, the scales are seen to stand out boldly from the surface at the upper part of the scale, they also project beyond the edge. These insects abound in damp cellars, where they may be seen running or skipping on walls.
Scale of Morpho Menelaus. A scale of this splendid butterfly, when magnified five hundred diameters, exhibits strongly marked longitudinal and very delicate transverse strip, the former frequently bifurcates. A half inch achromatic object glass should show them readily. Several other tests, besides those above enumerated, have been discovered by modern microscopes, which our confined limits prevent us from noticing.
Hipparchia Janira, the common meadow brown butterfly. The scales of this butterfly consist of longitudinal strip, with a number of brown spots of irregular shape. When the magnifying power is increased to one thousand two hundred diameters, the brown cells are made more evident.
