134 



A. ASHE ON TWO-SPEED FINE ADJUSTMENTS. 



carrying the body of the microscope, and compressing the spring 

 (g). The lower head (b) has, in this instance, simply the action 

 of a washer placed between A and c. 



A left-handed rotation of a, like the loosening of a nut^ 

 permits the expansion of the spring to force upwards both c 

 and B, until stopped by a. 



When on the contrary b is turned to the right, it screws 

 itself into c, thereby tending to leave a space between a and 

 b. But this space is at once taken up by the expansion of 



B 



F — 



Fig. 2. 



the spring, thereby raising the cap c together with b, and 

 consequently the focus. 



It will be clearly seen from the foregoing that although both 

 screws are right-handed, the milled heads attached to them pro- 

 duce opposite motions in the adjustment, hence the simultaneous 

 rotation of the two heads in the same direction w^ill give the 

 same effect as a differential screw, provided the screws are not 

 of the same pitch. 



In practice a might have 25 threads and b 27 to the 

 inch. A single revolution of a would then correspond closely to 

 a movement of the focus of one millimetre, whilst if both were 



