the emerging- rays coming from the top of the 

 cover glass are so refracted that they fail to touch 

 the front lens of the objective and are therefore 

 lost. In the water immersion, however, the rays 

 being less bent, more of them reach the front lens 

 and are passed through the objective. With the 

 oil immersion the only refraction is that which 

 takes place at the lower surface of the cover and 

 from that point the rays are carried uninterruptedly 

 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 lost might be utilized, but as the radius 

 of the front lens would have, to be increased we 

 know that its focus and magnifying power would 

 be decreased. This, however, would be detrimen- 

 tal, inasmuch as, according to the law which has 

 been determined and promulgated by Prof. Abbe, 

 the capacity of an objective is determined by the 

 ratio between its focal length and the diameter of 

 the emergent pencil at the point of its emergence, 

 from the back of the objective, or in other words, 

 it is " the sine of half the angle of aperture multi- 

 plied by the refractive index of the medium 

 between front of objective and cover." This has 

 been called by Prof. Abbe the Numerical Aperture, 

 and is now commonly used to designate the effici- 



