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When first setting up the instrument the largest 

 opening of the diaphragm should be used and if 

 the light is found too intense, it may be reduced 

 by bringing into position a smaller stop, or in case 

 of the iris diaphragm, by reducing the size 

 of opening. The iris diaphragm is infinitely 

 superior to the revolving or cap diaphragm as 

 these have fixed openings, whereas with the former 

 any intermediate gradation may be obtained. 



It is now generally conceded that observations 

 may be made with the microscope to any extent 

 without any detrimental results to the eyes, pro- 

 vided however, that the conditions of light are just 

 right. It is a good rule to follow, to use the least 

 illumination which will show the structure being 

 studied and it may also be safely accepted that if 

 the eye tires or feels uncomfortable, that 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 

 structure which cannot be seen with central illu- 

 mination. 



How to Focus. A safe method to follow and 

 one which is generally in vogue by all careful 

 manipulators, is to focus itpwards that is, bring 

 the objective closer to the object than its working 



