20 



Note. 



ciently great to warrant the trouble and expense of this extra 

 fitting. If, for instance, we could get speeds of, say, 5 ^ and ^ 

 for a revolution of each of the respective micrometer-screws, then 

 the full benefit of a two-speed fine adjustment would be at once 

 apparent. Now, as the speed of an ordinary coarse adjustment 

 may be taken as * 65 in. for each revolution of the coarse adjust- 

 ment pinion, a ^ will be 19^ times slower than the coarse ad- 

 justment, and 16£ times faster than the finest. (If the inter- 

 mediate motion had a movement of • 036 in. for each revolution 

 it would be 18 times slower than the coarse, and 18 times faster 

 than the finest adjustment.) 



But in the Microscope exhibited here the intermediate adjust- 

 ment had a speed of T ^ in. for each revolution, which is three 

 times slower than the fine adjustment of the ordinary Continental 

 Microscope. 



Fig. 2 illustrates the proposed alteration, which, like the 

 Males-Watson, consists of a lever of the second order engaging 







D 



III; Hi 



■in 



rt 



1 



B 



O x 



-52 



F 



n 



TX 



Fig. 2. 



with one of the first ; the springs at E and ¥ to keep the levers 

 A and B in close contact with the micrometer-screws C and D are 

 not shown. 



If, for example, the lever A had a ratio of 2:1, and B 4 : 1, and 

 the micrometer-screw C had 15 threads, and that at D 65 threads 

 to the inch, very suitable speeds would be obtained. Both micro- 

 meter-screws should be left-handed. 



