30 PRACTICAL HISTOLOGY. 



or withdrawn to any desirable distance. Nevertheless, it 

 entails loss of time to shift the tubes, and for ordinary 

 work nothing is so convenient as the circular plate. Its 

 margin should be notched like a mill-headed screw, and 

 there should always be a spring catch for arresting the rota- 

 tion of the plate when any one of its apertures comes 

 exactly under that of the stage. 



45. Employment of Daylight. The best light for 

 microscopical work is that reflected from a white cloud. 

 The light from the blue sky is not so good, and the light 

 from gray clouds is about the worst. The window for 

 microscopical work should, if possible, face that part of the 

 sky opposite the sun at the time the microscope is used. 

 On this account a north light is that which is most useful. 

 If the microscopist is compelled to work at a window into 

 which the sun shines, he should cover it with a white calico 

 blind, for direct sunlight must never be employed. Good 

 daylight is almost always to be preferred to artificial light ; 

 it often happens, however, that the latter must be employed. 

 The mode of obtaining it will be considered in 51. 



It is of great importance to protect the eye from 

 extraneous light. For this purpose a black paper or card- 

 board shade should be placed in front of the eye, so that 

 as little light as possible may reach it, save through the 

 microscope. With such a simple precaution one can work 

 much longer without fatigue, and the sharpness of the 

 microscopic image is much improved. 



THE MECHANICAL PARTS OF THE COMPOUND 

 MICROSCOPE. 



46. The Supports. The pedestal of the microscope 

 may be of a tripod or of a horse-shoe form, it matters 

 little, provided that perfect steadiness be obtained. 



The stage (L, Fig. i) should be a little broader than the 

 length of a slide (3 inches), and it is very important that it 

 be not too high. It ought to allow the forearm to rest on 

 the table while the thumb and forefinger are moving the 

 slide to and fro upon the stage. It is singular that English 





