ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 519 



Fig. 121 (from a drawing communicated by Mr. Swift) shows the 

 construction of average size. 



New Fine Adjustment.*— Mr. E. Gundlach, of Eochester, U.S.A., 

 has introduced a device by means of which an extremely slow, fine 

 adjustment can be obtained in addition to the ordinary coarse screw 

 movement. It is described as follows : — 



" In working high powers, microscopists have felt the need of a 

 finer adjust cent than the ordinary micrometer-screw, which cannot be 

 made much finer and still be durable enough. This need is now sup- 

 plied by the eambination of two screws which give a resultant motion 

 equal to the diderence in the threads employed. One of these screws 

 is a little coarser than the ordinary micrometer-screw, and may be 

 used alone as a fine adjustment, and a change can be made instantly 

 from this to the finer motion. Either is given by one milled head 

 located in the usual position of the fine adjustment screw-head on 

 Gundlach's Microscopes, and the change is made by turning a smaller 

 clamping screw having its head over the former. By tightening the 

 clamping screw, the adjustment is in order for the work of the com- 

 bination ; by loosening, for that of the coarser screw only. As the 

 thread of this is a little coarser than the ordinary micrometer-screw, 

 it alone gives a better motion for medium powers than the fine 

 adjustment in common use, a second advantage of the invention." 



Oil-immersion Objectives with Correction Adjustment. — With 

 regard to the advantage or otherwise of adopting the correction- 

 adjustment to oil-immersion objectives, Professor Abbe appears to 

 have thought j the errors of centering, likely to be introduced by the 

 movable mounting required for the adjustment, would be so sensibly 

 felt with the high apertures for which the formula was designed, that 

 it would probably be advantageous not to provide the adjustment, but 

 to mount the lenses in fixed settings. Dr. Woodward, of Washington, 

 has also expressed his approval of the fixed settings on the ground 

 that the formula does not practically need a correction-adjustment. 

 The photographs of difficult test-objects produced by him with the 

 Abbe-Zeiss oil-immersions prove, at any rate, that the particular 

 objectives referred to certainly yielded excellent definition with fixed 

 settings. 



The objectives recently made on this formula by Seibert and 

 Krafft, of Wetzlar, are also similarly mounted, as are the most 

 recent ones of Gundlach, of Eochester, N.Y. 



In England Mr. Stephenson ^ has acquiesced in the non-adoption 

 of the correction-adjustment for these lenses. 



On the other hand, Mr. Tolles, of Boston, Mass., and Mr. Spencer, 

 of Geneva, N.Y., have mounted their best objectives on this formula 

 with adjustment. It is also known that Messrs. Powell and Lealand 

 prefer the mounting with correction-adjustment, § which they have 



* Amer. Natural., xiv. (181) p. 346. 



t See tills Journal, ii. (1879) p. 261 note. t Il^i^l- '• (1878) p. .51. 



§ Ibid., iii. (1880) p. 1084. 



