( 8 ) 



I. Numerical Aperture Table. 



The " Apebtube" 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 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 aU 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 w sin u for any particular case is the "numerical aperture" of the objective. 



Example. 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), 167° (air), 142° (water), 130° (oil). 

 Their actual apertures are, however, aa '80 -98 1-26 1-38 or their 



numerical apertures. 



