mersion lens differs from the so-called "dry" lens 

 in that its higher magnifying power is combined 

 with remarkable resolving properties. This latter, 

 however, is achieved in a rather peculiar way: only 

 when a drop of cedar oil (or similar substitute) is 

 applied to fill the extremely small space which 

 exists between the lens and the object. 



Let me explain. As lenses increase in magnify- 

 ing power, they decrease in physical dimensions. 

 Consequently, in the oil-immersion objective the 

 effective aperture of the front lens is little more 

 than a thirty-second of an inch in diameter. Ordi- 

 nary white light being radiated from an object 

 equally in all directions, it is evident that the more 

 rays collected by the lens, the more distinct will be 

 the image. Thus the aperture of a lens, in relation 

 to its distance from the object, determines the qual- 

 ity of definition. But there is another feature which 

 has a very important bearing on the question of 

 the effectiveness of the oil-immersion lens. We 

 know that, according to the laws of refraction, as 

 the light rays leave an object, if the medium 

 through which they pass becomes more dense, the 

 divergent rays become more bent — that is, the 

 refraction becomes greater. So, cedar oil, happen- 



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