CHAPTER XXVIII 



USING THE MICROSCOPE 



To use any instrument effectively, one should know something 

 about its structure. The optical principles of the microscope are pre- 

 sented in any textbook of physics. Excellent practical hints are given 

 in two booklets published by the leading American optical companies. 

 These booklets tell the beginner how to set up the microscope, how 

 to keep it in order, and give directions concerning illumination, dry 

 and immersion objectives, mirror, condenser, diaphragm, and various 

 other things (Fig. 137). They were written for advertising purposes, 

 but since they advertise by giving directions for securing the best re- 

 sults with the microscope, the information is very practical. The 

 Spencer Lens Company, Buffalo, New York; and the Bausch and Lomb 

 Optical Company, Rochester, New York, furnish these booklets free 

 of charge. 



A cheap microscope with a 16 mm. objective and one ocular can be 

 used for examining preparations while they are wet with alcohols, oils, 

 or other reagents. If it is necessary to use a better instrument for such 

 work, cover the stage with a piece of glass — a lantern slide is of about 

 the right size — and be extremely careful not to get reagents upon the 

 brass portions. 



MICROMETRY 



One should learn to make some estimate of the size of a microscopic 

 object just as one can make an estimate of the size of larger objects; 

 but, in addition, everyone who uses a microscope should become able 

 to determine just how much it magnifies and should learn how to 

 measure microscopic objects. In any measurement one should note 

 the tube length, which is usually 160 mm. Some companies still make 

 the tube so short that it must be pulled out to reach the desired length 

 of 160 mm., even when the nosepiece is in place. Where there is no 

 nosepiece, the draw tube is simply pulled out until the length is 160 

 mm. (Fig. 138). Where a nosepiece is used, its height should be 

 measured, and the draw tube should be pushed in a distance equal to 

 the height of the nosepiece. 



There are in general use two practical methods of measuring micro- 



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