( 6 ) 



I. Numerical Aperture Table, 



The " ArKUTlTRK" of an optl&il Instramont Imilcates its greater or less capacity fi)r receiving rays from the object and 

 transniiuin(5 tberu to the Image, and the aperture cf a .Microsc())>e olij'.-ciive is tlierel'urc aJtomiined by the ratio 

 between iis focal length and the diameter of the emergent j»cncll at the plane of lu» emergence— Uiat ia, Uio utilized 

 diameUr of a single-lens objective or of the back lens of a compound objective. 



This ratio ia expressed for all rae<lia and In all cases by n sin u, n being the refractive Index of the medium and u the 

 semt-augle of aperture. The value of n sin u for any particular case is the " numerical aperturt " of the objective. 



ExAMPiE. — 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 aperture? are, however, as '80 '98 1'26 1-38 or their 



niunerical apertures. 



