CH. Ill] TESTING THE MICROSCOPE 97 



lumination. To avoid the discomfort of facing the light a shield (fig. 

 33) is very useful. For advanced students and private workers a desk 

 like that shown in fig. 58 is very convenient. 



TESTING THE MICROSCOPE 



163. Testing the microscope. To be of real value this must 

 be accomplished by a person with both theoretical and practical 

 knowledge, and also with an unprejudiced mind. Such a person is 

 not common, and when found does not show overanxiety to pass 

 judgment. Those most ready to offer advice should as a rule be 

 avoided, for in most cases they simply "have an ax to grind," and 

 are sure to commend only those instruments that conform to the "fad " 

 of the day. From the writer's experience it seems safe to say that the 

 inexperienced can do no better than to state clearly what he wishes 

 to do with a microscope and then trust to the judgment of one of the 

 optical companies. The makers of microscopes and objectives guard 

 with jealous care the excellence of both the mechanical and optical 

 part of their work, and send out only instruments that have been care- 

 fully tested and found to conform to the standard. This would be 

 done as a matter of business prudence on their part, but it is believed 

 by the writer that microscope makers are artists first and take an 

 artist's pride in their work; they therefore have a stimulus to excel- 

 lence greater than business prudence alone could give. 



What has just been said does not by any means imply that the 

 purchaser of a microscope should blindly accept anything which is 

 offered him. It simply means that if one has no knowledge of a 

 microscope one can hardly pass expert judgment upon it. 



164. Mechanical parts. All of the parts should be firm, and 

 not too easily shaken. Bearings should work smoothly. The mirror 

 should remain in any position in which it is placed (fig. 25). 



165. Focusing adjustments. The coarse or rapid adjustment 

 should be by rack and pinion and work so smoothly that even the 

 highest power can be easily focused with it by an experienced observer. 



This coarse adjustment is liable to work too hard or too easily. If 

 it works too hard, the bearings of the pinion are too tight or the gliding 

 surfaces are sticky and not properly lubricated. If the bearings are 



