any detrimental results to the eyes, provided, however, 

 that the conditions of light are just right. It is a good 

 rule to follow, to use as small an amount of illumination 

 as will comfortably show the structure which is being 

 studied, and it may also be safely accepted that if the 

 eye tires or feels uncomfortable the light should be 

 moderated. 



Illumination is either : 



Central or axial, when the center of the mirror 

 is in the optical axis, or 



Oblique, when the mirror is swung to one side 

 which, in objectives of wide aperture, will disclose struc- 

 ture that cannot be seen with central illumination. 



To Focus an objective is to adjust its relation to 

 the object so that a clear image is obtained. To 

 properly and safely focus an objective is a matter of 

 great importance and a certain line of procedure should 

 be followed, which in time should become habitual. 

 Focusing should involve no danger to the front lens 

 of the objective or to the cover glass by their coming 

 in violent contact. With the low power objectives, in 

 which the working distance is great, there should be 

 little danger ; with the higher power objectives, in 

 which the working distance is so small that the front 

 of the objective is very close to the cover glass, there 

 is considerable. 



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