( 8 ) 

 I. Numerical Aperture Table. 



The " Aperture" of nn optical instrument indici\tes its greater or loss capaolty for receiving rnys from the object iind 

 transmittlDg. them to the image, and the aperture of a Microscope objective is therefore determined by llic 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 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 m for any particular case is the "numerical aperture" of the objective. 



r,F,. The apertures of four objectives, two of which are dry, one water-immersion, and one oil-immersion, 



iild be compared on the awjular aperture view as follows:— 106° (aul, 157° (air), 112° (water), 130° (oil), 

 ctiial apertures are, however, as -80 -98 1-26 1-38 or their 



EXAMPI, 



woii.„ . i „ 



Their actual apertures are, however, as 

 numerical apertures 



