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I. Numerical Aperture Table. 



The " Aperture" of an optical instrument indicatesits 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 acompound 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 aparture. 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-immersion, and one oil-immersion, 



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



Their actual apertures are, however, as '80 '98 1-26 1-38 or their 



numerical apertures. 



