74 LIGHTING AND FOCUSING [ CH. II 



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

 a microscope and then trust to the judgement of one of the optical companies. 

 The makers of microscopes and objectives guard with jealous care the excel- 

 lence 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 stand- 

 ard. 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. 



\ 130. 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 

 micoscope 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 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 micro- 

 scope entirely. It has been much improved recently. 



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 absolutely no motion except in the direction of the optic axis, other- 

 wise the image will appear to sway even with central light. Compare the ap- 

 pearance 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 {\ 88). 



§ 131. 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 advant- 

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

 microscope 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). 



LABORATORY AND HIGH-SCHOOL COMPOUND 

 MICROSCOPES 



\ 132. Optical Parts. — A great deal of beginning work with the micro 

 scope in biological laboratories is done with simple and inexpensive apparatus. 



