ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



609 



any convenient length. By moving the handle sideways the sleeve is 

 made to seize upon the shank of the milled head, and the long handle 

 allows of a very delicate motion being imparted to the pinion. With a 

 well-cut rack the author finds even high-power objectives can be satis- 

 factorily focussed. 



This form of fine adjustment by means of a loose lever attached to 

 the coarse adjustment pinion, was described by Messrs. Smith and Beck 

 at the Microscopical Society of London on October 9th, 1861.* A very 

 similar construction by Ladd is also figured in the third edition of 

 Carpenter, p. 81, fig. 27 (1862). Mr. Beck states in a foot-note that 

 Mr. Brookes (Charles Brooke ?) was the first to recommend this kind of 

 slow motion. 



Males-Watson Two-speed Fine Adjustment. — The essential feature 

 in this new two-speed fine adjustment is a lever similar to that regularly 



Fig. 114. 



fitted to Watson's Microscopes. The fulcrum of this is at D, and the 

 lever is worked by the milled head A on the point E. By using a coarse 

 thread for the screw a speed of ^th of an inch for each complete 

 rotation is produced. The second and slower speed is obtained by 

 another lever fitted to the first at the point F, which is acted on by the 

 screw B at the point G. A somewhat finer screw is used for this, and 

 the combination of the levers yields a movement as slow as ^th of an 

 inch for each complete turn of the milled head. Any desired ratio of 

 speed can be obtained by altering the pitch of the threads of the micro- 

 meter screws. Fig. 113 shows the adjustment fitted to a Watson's Van 

 Heurck Microscope, and Fig. 114 gives a sectional view of the working 

 parts. 



* Trans. Mic. Soc, x. (18C2) p. 11, pi. . r >. 



