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I. Niimerical Aperture Table. 



The " Apertube" of an optical instrument indicates its greater or less capacity for receiving rays fioin the object and 

 transroittlng them to tlie image, and the aperture of a Microscope objective is therefore determined by the ratio 

 between its focal length and tbe 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 t>eing tbe refractive Index of tbe medium and m the 

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



!;xAMi"LF.. The apertures of four objectives, two of which are dry, one water-immersion, and one oil-immersion, 



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



Their actual apertui«8 are; however, as -80 -98 1'26 1-38 or their 



numerical apertures. 



