98 TESTING THE MICROSCOPE [Ch. Ill 



too tight, loosen the screws very slightly; if the bearings are not lubri- 

 cated or the surfaces are covered with sticky oil, wet a cloth with a 

 good lubricating oil and rub the gliding surfaces well. This will clean 

 them and lubricate them at the same time. 



If the tube runs down too easily the bearings of the pinion are too 

 loose and the screws should be tightened a little. 



§ 166. 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 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; 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 (§ 116). 



§ 167. 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 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 parts of his microscope. 



If one can compare a new or unfamiliar microscope with one with 

 which there is entire familiarity, a very good estimate can be made. 

 The first principle is to use some microscope with which one is familiar 

 and to use microscopic preparations of which one knows the structure; 

 then a fair judgment can be made of the excellence of the performance 

 of the new instrument. If there seems to be any defect in the image, 

 make sure 



(i) that the lighting is good; 



(2) that the proper aperture of the objective is being used and that 

 the condenser is centered (§ 104); 



