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I. Numerical Aperture Table. 



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

 transmitting them to tbe image, and the aperture of a Microscope objective la therefore determined by the ratio 

 between its focal length and the diameter of the emergent pencil at the plane of its emergence — that is, the utilized 

 diameter of a single-lens objective or of the back lens of a compound objective. 



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

 semi-angle of aperture. The value of ra sin u for any particular case is the "numerical aperture-" of the objective. 



