MICROSCOPE 263 duced microscopic objectives that surpassed any then con structed on the Continent, while the subsequent adoption of the same principles by French and German opticians, as also by Professor Amici of Florence, soon raised their objectives to a corresponding level. It has proved more advantageous in practice to make the several components of an achromatic objective correct each others aberrations than to attempt to render each perfect in itself; and the mode in which this is accomplished will vary with the focus and angular aperture given to each combination. Thus, while a single " telescopic triplet " answers very well for the lowest power usually made (4 inches focus), and the same plan may be used though at the sacrifice of angular aperture for objectives of 3 inches, 2 inches, and even 1 inch focus, the best per formance of these powers requires the combination of two doublets. And, while this last system also serves for objectives of inch and inch of low angle, a third com ponent is required for giving to these objectives the aperture that renders them most serviceable, as well as for all higher powers. Instead of combining three achromatic doublets, however, many makers prefer placing in front a plano-convex of crown, and adding a third lens of crown to the doublet at the back, still using a doublet in the middle, the whole combination thus consisting of six lenses, four of crown and two of flint. Further, Mr Wenham has shown that the whole colour-correction may be effected in the middle by interposing a double concave of dense flint between two double-convex lenses of crown, the back lens, as well as the front, being then a plano-convex of crown, making five lenses in all. This plan of construction, though suitable to objectives of moderate angular aperture, and advantageous in regard to comparative simplicity and economy of construction, does not seem so well adapted for objectives to which the largest attainable aperture is to be given, these being usually constructed with a triplet in front, a doublet in the middle, and a triplet at the back, so as to consist of eight separate lenses. And the first-class constructors of achromatic objectives in the United States usually place in front of these, in their highest powers, a single plano-convex of crown, by the addition of which a greater working distance can be obtained. But, as every such addition increases the liability to error from imper fections in the centring and grinding of the lenses (as well as loss of light by the partial reflexion of oblique rays from their surfaces), it is obvious that the most exact workmanship, involving a proportionate costliness, is required to bring out the full effect of such complex con struction. And where angular aperture is regarded as the quality of primary importance it will be usually found preferable to have recourse to objectives constructed on either the " water " or the " oil " immersion system, to be presently described. The great increase thus attained in the perfection of the corrections of microscopic objectives for both spherical and chromatic aberration of course rendered it possible to make a corresponding increase in their angular aperture. The minute scales of the wings of butterflies and other insects were naturally among the objects much examined ; and it was soon perceived that certain lines and other markings became clearly discernible on these scales with objectives of what was then considered large angle which were utterly undistinguishable with non-achromatized microscopes (however high their magnify ing power), and very imperfectly shown under achromatic objectives of small angle. Hence these scales came to be used as " test-objects," for judging of the " definition " and "resolving power" of microscopic objectives, the former property consisting in the clearness, sharpness, and freedom from false colour of the microscopic images of boundary Fio. 5. Scale of Morpho menelaus. lines, and depending on the accuracy with which the rations are corrected, while the latter term designates that power of separating very closely approximated markings which is now known to be a " function " of aperture. The insect-scales formerly most valued for these purposes were those of the Morpho menelaus (fig. 5) and the similarly lined scales of the Polyommatus argiis (azure-blue), the "battledoor" scales of the same butterfly (fig. 6), the ribbed scales of the Lepisma saccharina (sugar-louse), and the minute and peculiarly marked scales of the Lepidocyrtus curvicollis (fig. 7), commonly known as the Podura. The writer recollects the time when the satisfactory " resolu tion " of the first three of these tests was considered a sufficient proof of the goodness of even high-power objectives, and when the Pcx/wra-markings, if visible at all, could only be dis tinguished as striae. The further opening-out of the aperture, however, enabled these striae to be resolved into rows of " exclama tion marks " ; and, while there is still some uncertainty as to the precise structure of which these markings are the optical expression, practical op ticians are generally agreed that the Podura-sc&lQ is very useful as a test for definition, with even the highest objectives, though it only serves as a test for a very moderate degree of re solving power. For the latter purpose it has been completely superseded by the closely approximated markings of the silicified envelopes of certain FlG diatoms (which, however, show them selves in very different aspects accord ing to the conditions under which they are viewed, figs. 8-11), and also by lines artificially ruled on glass, as in Nobert s "test-plate," the number of lines in the nineteen bands of which is stated by M. Nobert to range from 1000 to 10,000 to a Paris line, while Dr Royston Pigott gives the numbers in an English inch as 1 1,529 to the inch in the first band, and 112,595 in the nineteenth. This last 6. Battledoor Scale of Polyomma tus argus. dimension (as will afterwards appear) approaches the minimum distance at which such markings are theoretically separable by any magnifying power of the micro scope. The enlargement of the angle of aperture of microscopic ob jectives and the greater complete ness of their corrections, which were obtained in the first in- Fio. 7. Test-Scales of Podura stance by the adoption of Mr gtM^tS Lister s principles, and were de- SC aie ; B, small scale, more monstrated by the resolution of faintly marked. the test-objects then in use, soon rendered sensible an imperfection in their performance under certain circum
stances, which had previously passed unnoticed; and the