349 



Note on a Method of using Abbe's Apertometer. 



By Frederic J. Cheshire. 



(^Rcad May 16th, 1902.) 



On pp. 395-6 of the 8th edition of '-The Microscope," 

 by Oarpenter-Dallinger, a method of using Abbe's apertometer 

 is described, which consists in placing it upon the graduated 

 rotating stage of a microscoj^e, and using as an indicator the 

 edge of a fixed lamp flame. The stage is rotated to bring the 

 indicator from the centre to the edge of the field, and the N.A 

 is calculated from the angle through which the stage is rotated, 

 upon the supposition that it is equal to the semi-angle of the 

 maximum cone of light in the glass which the objective can take 

 up. This method, however, if employed without certain pre- 

 cautions beicg taken, is subject to an error, which under certain 

 conditions becomes serious. 



Let the figure show a plan of the apertometer in use in the 

 way described. Let L be the lamp and D its distance from the 

 centre O' of the focussing disc. 



Now the centre of the circular edge of the apertometer is not 

 the centre of the focussing disc, but a point O midway along the 

 back edge. Suppose that the objective, being tested, can take 

 up the cone of light from the glass, the semi-angle of which is a, 

 the limiting ray then must pass along the line OB, making 

 an angle a with the line OL, before striking the inclined 

 reflecting edge of the apertometer. Now about 0' describe a 

 circle with a radius D intersecting the line OB produced at L', 

 and draw O'L'. In rotating the apertometer about an axis 

 passing through the point O', i.e., about the optical axis of the 



JouRN. Q. M. C, Series II. — No. 51. 23 



