PROPERTIES OP LENSES. 19 



at the focal point of the objective, has been super- 

 seded by the more correct and convenient method 

 of calling the capacity which an objective possesses 

 for collecting the whole pencil of rays from an object 

 in air, Unity, then commencing with the lowest 

 numbers this reaches as far as 1'52 for oil immer- 

 sions. This is called the Numerical aperture and 

 NA is written after the numbers to distinguish it. 

 It may easily be found by multiplying the sign of 

 the semi-angular aperture by the refractive index of 

 the fluid used. 



The angular aperture may be increased by using 

 oblique light, which enables the rays illuminating 

 the object to pass through at a greater angle, or an 

 achromatic condenser, which increases both their 

 number and obliquity. 



This part of the subject would be incomplete with- 

 out reverting to the views enunciated by Professor 

 Abbe of Jena, which have placed the old theories of 

 Microscopic vision on an entirely new footing. Ac- 

 cording to his demonstrations the images produced 

 by minute objects, small multiples of wave lengths, 

 are imaged in an entirely different way from those 

 produced by coarse objects. Objects less than ^rgW 

 of an inch are imaged by the diffracted rays pro- 

 duced by the action of the minute structure. The 

 more diffracted rays reach the eye the truer the 

 structure. The more minute the object the more 

 the diffracted rays are spread, therefore the larger 

 the angular aperture the greater the quantity of 

 diffracted rays transmitted and the truer the result- 



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