MANIPUL/ITION OP THE MICROSCOPE. 



33 



standard. Such changes in adjustment are necessary only 

 in the case of dehcate work. 



5. Care of the Eyes. — Much of the difficulty some- 

 times experienced by beginners with the microscope may 

 be avoided by attention to three points mentioned above 

 — the cleanliness of the lenses, a clear but not excessive 

 illumination of the field, and a proper manipulation of 

 the fine adjustment. In no case should the observer 

 strain his eyes in the attempt to study what he cannot 



Fig. 22. — Micrometer Calipers. 



see clearly. If the object is not distinct, there must be 

 something wrong which should be remedied. 



The distance of the eye above the eyepiece should 

 vary with the magnification. For each combination of 

 lenses there is a certain eye-point at which a maximum 

 number of rays is most closely concentrated, and above 

 or below, the size of the field will be reduced, and colored 

 shadows will appear. The higher the power the nearer 

 the eye-point approaches to the eyepiece. 



It is best to accustom oneself to the use of both eyes 

 alternately, and to acquire the habit of keeping open the 

 eye which is not over the instrument. At first the atten- 



