192 THE USE OF THE MICROSCOPE 



23. It is usually a mistake not to have all objectives 

 and eyepieces by the same maker. 



24. It is wrong (Nelson) not to use a 14-millimeter 

 diaphragm in the drawtube of the monocular microscope. 



25. With a 60 apochromatic objective (of 1.3 or 1.4 

 aperture) the tube length must be adjusted to 160 milli- 

 meters within less than a millimeter (with correct cover 

 thickness). 



26. An objective with correction collar should not be 

 regularly used by setting the collar at one point (say 15). 

 It will pay to learn to measure the covers. 



27. In using objectives corrected for 170 millimeters 

 (Leitz), on a microscope with fixed tube length of 160 

 milhmeters, collars 10 millimeters deep may be fitted to the 

 eyepieces, to get optimum images. 



28. An achromatic condenser, made for a distant source, 

 loses much aperture if used with a near lamp, without an 

 appropriate achromatic correcting lens being fitted below. 



29. Do not take photographs with a microscope objective 

 alone, if optimum results are required. 



30. Do not take photographs with Huyghenian, or 

 compensating, or other eyepieces used for observation, 

 if optimum results are required. The special projection 

 eyepieces or the homal eyepieces accord with optical 

 laws. (If an eyepiece is nevertheless used, it is better 

 to focus by pulling out the tube, than by raising the 

 objective.) 



MICROSCOPICAL MISHAPS 



1. If the low-voltage lamp burns out too quickly, and the 

 transformer and main voltage are right, it is probably due 

 to the lamp being turned on again too soon to have cooled 

 off. 



2. If the light is somewhat too dim with the high power, 

 see if the ground glass is centered with respect to the 

 incandescent ribbon, and is close enough. In a pyrex 

 lamp it can be quite close. 



