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II. NUMERICAL APERTURE TABLE. 



The " APERTtTEE" of an optical instrument indicates its greater or less capacity for receiving rays from the object and 



transmitting them to the image. 

 In the y«Zescqpe-objective its aperture Is determined by the diameter of the admitted pencil at ite entrance — that Is, the 



absolute (utilized) diameter of the objective. 

 In the Mcroscope-objective its aperture is determined by the ratio between the focal length and the diameter of the 

 emergent pencil at the plane of its emergence— that is, the relative diameter of the single-lens objective or of the 

 back lens of a compound objective. 



This ratio is expressed for all media and all cases by n sin u, n being the refractive Index of the medium and 

 u the semi-angle of aperture. The values of n sin u for any particular cases are the "numericaX aper- 

 tures" of the objectives. 



