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I. Numerical Aperture Table. 



Tlie " Aperturb" of an opftcal Instrument indicates its greater or less capacity for receiving rays from the object and 

 transiuittlug lliem to tlie image, and the aperture of a Microscope objeclive is tlierefore determined by tlie ratio 

 between iis focal length aiid the diameter of the emergent pencil at the plane of its emergence— that is, the ntilized 

 diameter of a single-lena objeoiive or of the back lens of a compound objective. 



This ratio is expressed for all m.^dia and in all cases by n sin u, n being the refractive index of the medium and u t'le 

 semi-ungre of aperture. The value of ra sin u for any particular case is the •'numerical aperture" of the objjctive. 



ExAjiri-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 1-33 or their 



numerical apertures. 



