CH. If] LIGHTING AND FOCUSING 63 



sities of special cases. It is a great advantage to sit facing the window 

 if daylight is used, then the hands do not constantly interfere with 

 the illumination. To avoid the discomfort of facing the light a screen 

 like that shown in Fig. 59 is very useful (see also under lighting, 



62). 



TESTING THE MICROSCOPE 



| 118. Testing the Microscope. To be of real value this must be accom- 

 plished 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 over anxiety 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 is 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 carefully 

 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 excellence greater than business prudence alone could give. 



2 119. 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. 



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. In no case should it work so easily that the body of the microscope is 

 liable to run down and plunge the objective into the object. If any of the above 

 defects appear in a microscope that has been used for some time, a person with 

 modjerate mechanical instinct will be able to tighten the proper screw, etc. 



The Fine Adjustment is more difficult to deal with. From the nature of its 

 purpose unless it is approximately perfect, it would be better off the microscope 

 entirely. It has been much improved recently. 



It should work smoothly and be so balanced that one cannot tell by the feel- 

 ing when using it whether the screw is going up or down. Then there should be 

 absolutely no motion except in the direction of the optic axis, otherwise the 

 image will appear to sway even with central light. Compare the appearance 

 when using the coarse and when using the fine adjustment. There should be no 

 swaying of the image with either if the light is central ($ 77). 



| 120. Testing the Optical Parts. As stated in the beginning, this can be 

 done satisfactorily only by an expert judge. It would be of very great advantage 

 to the student if he could have the help of such a person. In no case is a micro- 

 scope to be condemned by an inexperienced person. If the beginner will bear in 

 mind that his failures are due mostly to his own lack of knowledge and lack of 

 skill ; and will truly endeavor to learn and apply the principles laid down in this 

 and in the standard works referred to, he will learn after a while to estimate at 

 their true value all the pieces of his microscope. (See Ch. X). 



