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I. Numerical Aperture Table. 



The " Apeettjee" 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 cf 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 w sin m, n being the refractive index of the medium and u the 

 semi-angle of aperture. The value of n sin u for anyparticular ceise is the "numericai aperture" of the objective. 



txAMPi.E. — 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. 



