fitted in its bearings, so that there will not be the least 

 side motion and this should be tested under the magni- 

 fying power of an objective. There should be no 

 sensation of the individual teeth coming in contact. 

 It is safe to assume that if the rack and pinion shows 

 either of the above defects, the instrument is faulty in 

 other directions as well. 



Fine Adjustment. Nothing in the microscope 

 will cause more aggravation than a faulty fine adjust- 

 ment. It should work with absolute smoothness and 

 with no side play in the screw. The body should 

 respond promptly, when moving the milled head 

 rapidly forward and backward and should not cause 

 any swaying of the image during observation. The 

 micrometer screw should be back of the pinion, not at 

 the front of the tube nor under the stage. 



Metal. Whether an instrument shall be of 

 japanned iron or lacquered brass is probably largely 

 determined by the amount of money to be expended. 

 So far as the intrinsic suitability of the metals is con- 

 cerned there is no difference. Brass, however, offers 

 the maker a better opportunity for displaying his 

 mechanical skill and while it is no doubt true that 

 many highly finished instruments are of poor work- 

 manship in their working parts, it is also a fact that a 

 well made instrument is always nicely finished. 



1 8.6 



