[Illustration: Fig. 114.--Lister’s Correction Collar, (in section).] No sooner had Ross constructed 1/4-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. [Illustration: Fig. 115.--The Continental Screw-collar Adjustment.]