Fig. 114.—Lister’s CorrectionCollar, (in section).
No sooner had Ross constructed ¼-inch achromatic objectives on Lister’s formula than he discovered an error which had hitherto escaped attention, viz., that the thinnest cover-glass of an object produced a considerable amount of refractive disturbance. A marked difference was observed in the image when viewed with or without a cover-glass. This difficulty was first met by the addition of a draw-tube to the microscope body. But as this also impaired the image, Lister overcame the difficulty by mounting the front lens of the objective in a separate tube made to fit over a second tube carrying the two pairs of lenses. This arrangement led up to his invention of the screw-collar adjustment, the mechanism for applying which is shown in [Fig. 114]. The anterior lens a at the end of the tube is enclosed in a brass-piece b containing the combination; the tube a, holding the lens nearest the object, is then made to move up or down the cylinder b, thus varying the distance, according to the thickness of the glass covering the object, by turning the screw ring c, thus causing the one tube to slide over the other, and clamping them together when properly adjusted. An aperture is made in the tube a, within which is seen a mark engraved on the cylinder, on the edge of which are two marks, a longer and a shorter, engraved upon the tube. When the mark on the cylinder coincides with the longer mark on the tube, the adjustment is made for an uncovered object; and when the coincidence is with the shorter mark, the proper distance is obtained to balance the aberrations produced by a cover-glass the hundredth of an inch thick; such glass covers are now supplied. The adjustment should be tested experimentally by moving the milled edge which separates or closes the combinations, and at the same time using the fine adjusting screw of the microscope. The difficulty associated with the cover-glass of old has, by the introduction of the homogeneous immersion system, been very nearly eliminated. There still remains, however, a disturbing amount of residual colour aberration in the achromatic dry objective, and for the correction of which Zeiss proposed mounting the several lenses on a method somewhat different to that so long in use in this country. [Fig. 115] shows an objective in which the screw-collar ring b b is made to adjust the exact distance between the two back lenses placed at a a. The value of the screw-collar is not questioned. It is difficult to obtain at all times cover-glasses of a perfectly uniform thickness; they will vary, and therefore perfect definition must be obtained, as heretofore, by adjusting for each separate preparation while the object is under examination.
Fig. 115.—The Continental Screw-collar Adjustment.
As early as 1842 the excellence of Andrew Ross’s achromatic objectives were acknowledged, and his formula for their construction was generally followed. No doubt many of these early objectives of his manufacture are still regarded as treasures. I possess a ½-inch and a ¼-inch, which I believe to be comparable with any achromatic objectives of the same apertures of the present day. These I have always found most serviceable for histological work.
In 1850 Mr. Wenham produced an achromatic objective of considerable achromatic value. This consisted of a single hemispherical front combination, shown in the accompanying enlarged diagram, [Fig. 116]. Wenham’s formula seems to have been generally adopted by Continental opticians, who sold these lenses at a reduction of price. In Paris, Prazmowski and Hartnack—I have had one of Hartnack’s earliest immersions in use for many years—brought this form of objective to greater perfection, and in 1867 Powell and Lealand adopted the single front combination system in their early water-immersion objective, whereby the focal distance was said to be “practically a constant quantity, while reduction of aperture by making the front lens thinner ensures a much greater working distance without affecting the aberrations, since the first refraction takes place at the posterior or curved surface of the front lens, the removal of any portion of thickness at the anterior or plane surface simply cuts off zones of peripheral rays without altering the distance—any space being filled by the homogeneous immersion fluid, or by an extra thickness of cover-glass.”[21]
Fig. 116.—A Single Front Combination formulated by Wenham for Messrs. Ross (enlarged).
Great improvements were brought about by R. B. Tolles, of Boston, 1874, in the objective, as well as in the optical and mechanical parts of the microscope, most of which, however, must be ascribed to the criticisms and suggestions of amateur workers skilled in the exhibition of test-objects—the late Dr. Woodward of Washington, for example, whose series of photographs of the more difficult frustules of diatoms have rarely been surpassed. Such results were due to improvements made in the optical part of the microscope at his suggestion. He came to the conclusion, arrived at about the same time by mathematical scientists, that increase of power in the microscope was only possible in two directions, the qualitative and the quantitative.
It was now that microscopists turned to the late Professor Abbe for assistance in perfecting the objective in the dioptric direction. This, he pointed out, must be looked for in further improvements in the art of glass-making.