20 PROPERTIES OP LENSES. 



Unity, then commencing with the lowest numbers 

 this reaches as far as 1*52 for oil immersions. 



This is called the Numerical aperture and NA is 

 written after the numbers to distinguish it. 



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. 



We cannot leave this part of the subject without 

 reverting to the views enunciated by Professor Abbe 

 of Jena, which have placed the old theories of Micros- 

 copic vision on an entirely new footing. 



According to his demonstrations the images pro- 

 duced by minute objects, small multiples of wave 

 lengths, are imaged in an entirely different way 

 from those produced by coarse objects. 



Objects less than ^-g^ of an inch are imaged by the 

 diffracted rays produced 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 resulting image as 

 regards the actual structure of the object. 



It has been recently remarked that microscopic 

 vision was totally distinct from macroscopic. 



First, because by placing a stop behind the objec- 

 tive that is decreasing the aperture, objects could 

 be made to assume different forms. 



