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other, but which takes in the same amount of 

 rays, it would appear that if the power alone were 

 to indicate the visibility of the object, the higher 

 one should show more detail, whereas they in 

 reality show equally well. 



Aperture, without the defining- word angular, 

 indicates a very important feature in an objective 

 and designates the beam or pencil of light which 

 passes out through the rear lens of an objective, 

 or in other words is the effective diameter of the 

 rear lens. Many objectives are made in which the 

 rear lens is larger in diameter than the beam of 

 rays which, coming from an object, can be trans- 

 mitted through it and while not particularly 

 detrimental, has no value except perhaps to lead 

 to a wrong conclusion in reference to angular 

 aperture when this is measured, as the excess of 

 image forming rays, called stray rays, may indi- 

 cate a greater angle than the objective really 

 possesses. 



Cover Glass. At this point we must introduce 

 another feature which we have not yet considered 

 but which is important in understanding this, the 

 most important property of an objective. As we 

 have shown, in describing the principle of refrac- 

 tion, rays which fall from air upon a surface of 

 glass, are bent out of their course or refracted. 

 The cover glass which is used on the object during 

 examination, or for its preservation, although 



