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II. NUMERICAL APERTURE TABLE, 



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

 transmitting them to the image, and the apertm-e 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 beuig the refractive index of the medium and u the 

 semi-angle of aperture. Xlie value of n sin u for any particular case is the " namerkal aperture" of the objective. 



ExAiirLE.— I'hp apertm-cs 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), ISi^-Kair), 142° (water), i:;o° (oil). 



Their actual iiiipvtures are, however, as -SO -98 1'26 1-38 or thi ir 



imTn:'rical :iperturc.<5. 



