Referring now to the diagrams and bearing in mind 

 the course of rays from an object through a cover 

 glass, we can follow out their action. In the case of a 

 dry objective, we see that a portion of the rays coming 

 from the top of the cover glass are so refracted that 

 they pass outside of the front lens of the objective and 

 are therefore lost. In the water immersion, however, 

 the rays are less refracted. With the oil immersion the 

 only refraction to be considered is that which takes 

 place at the lower surface of the cover as from that 

 point the rays pass without change of direction 

 through the cover, fluid, and front lens, as far as the 

 convex surface, where they are refracted and carried 

 through the objective. From a view of the diagram it 

 might appear that if the lens were enlarged in diameter 

 more of the extreme rays which are otherwise lost 

 might be utilized ; but as it is apparent that the radius 

 of the front lens must then be increased, we know that 

 this will increase the focal length and consequently 

 reduce the magnifying power. 



Numerical Aperture. The consideration of the 

 capacity of objectives to take up a greater or less 

 number of rays diverging from a given object leads 

 to the question as to whether any optical law may exist 

 in relation to the angular aperture of objectives, under 

 the different conditions of their use, either dry, or 

 with water, oil, or any other immersion fluid, and 

 Prof. Abbe has found that it is as follows : 



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