60 LIGHTING AND FOCUSING. [CU. II. 



TESTING THE MICROSCOPE. 



\ in. 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 un- 

 prejudiced mind. Such a person is not common, and when found, does not 

 show an 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 it seems safe to say that the inexperienced can do no 

 better than to trust to the judgment of one of the optical companies. The makers 

 of microscopes and objectives guard with jealous care the excellence of both Lhe 

 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. 



% 112. Mechanical Parts. — All of the parts should be firm, and not too easily 

 shaken. Bearings should work smoothly. The mirror should remain in any posi- 

 tion 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 

 moderate mechanical instinct will be able to tighten the proper screws, etc. 



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

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

 entirely. 



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

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

 solutely 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 (§ 73). 



\ 113. 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 the condemna- 

 tion of a microscope to be made 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 appendix). 



