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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 

 tiansniitting them to the image, and tlje aperture of a Microscope objective is tUerefora 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 bacli 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. 



Examplt;. — 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), 14^° (water), 130° (oil). 



Their actual apertures are, however, as -80 -98 1-26 1-3S or their 



uunifiical apertures. 



