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I. Numerical Aperture Table. 



Tbe " 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— Xhat is, tbe utilized 

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



This ratio is expressed for all media and in all cases by re sin u, re being the refractive index of the medium and u the 

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



■:xamvu-.. — The apertures of four objectives, two of which are dry, one water-immersion, and one oil-immersion, 



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

 riif-ir actual apertures are, however, as '80 -98. 1-2G 1-38 or their 



Their actual apertures are, however, as 

 numerical apertures 



