ADJUSTING THE MICROSCOPE 275 



and swing the high power objective into position, and then 

 while watching outside the microscope run the body tube down 

 until the objective all but touches the coverglass. It must be 

 very close indeed, and you must hold your eye on a level with 

 the stage to be certain about it and see that the objective does not 

 press against the coverglass. Now look through the ocular and 

 slowly draw the body tube up by turning the micrometer head of 

 the fine adjustment contra-clockwise, until the preparation comes 

 sharply into view. Starch grains of the potato have circular, 

 excentric striations, and if you can see these clearly with the high 

 power it is an indication that the microscope is adjusted to give 

 a good image. If the striations cannot be seen you may be sure 

 something is the matter. See that the mirror is adjusted to give 

 plenty of light and that the opening in the iris diaphragm is hardly 

 more than ^ in. in diameter, and see that the lenses of the 

 ocular and the front lens of the objective are perfectly clean. 

 These are the chief things to look to in securing a good image. 

 If the lenses need cleaning breathe upon them and quickly 

 wipe them off with a clean soft cloth; or dip the cloth into water 

 and wash the lenses and then dry them with gentle pressure 

 of a dry part of the cloth. A good tentative rule about the dia- 

 phragm opening is to have it about the size of the front lens of 

 the objective in use. If there is then not enough light the open- 

 ing may be made somewhat larger, but it will be seen that the 

 image becomes less sharp the larger the diaphragm opening is 

 made to be. Make it a rule from the beginning never to put 

 up with a poor image. 



There are some details about the working of a microscope 

 that need special mention. The tube length should be ad- 

 justed to 160 mm. by sHding the draw tube in or out (see Fig. 

 147). When a revolving nosepiece is used the tube length 

 should be measured from the eyelens to the lower edge of the 

 nosepiece. The coverglasses should be No. 2, excepting when 

 oil immersion objectives are employed with a working distance 

 so short as to require the thinner No. I coverglasses. It is to be 

 noted that the micrometer fine adjustment works through only a 



