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I. Numerical Aperture Table. 



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

 transmitting them to the image, and the aperture of a Microscope objective is 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 all 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 n sin u for any particular case is the *' numerical aperture " of the objective. 



Kxample. — The apertures of four objectives, two of which are dry, one water-immerfion, and one oil-immersion, 

 would be compared on the angular aperture view as follows : — 106 c (air), 157° (air), 142° (water), 130° (oil). 



Their actual apertures are, however, as 'SO -98 1-26 1-38 or their 



numerical apertures. 



