352 F. J. CHESHIRE ON A METHOD OF USING ABBE's APERTOMETER. 



Knowing the radius of the disc in millimetres and the refractive 

 index of the glass, it is then only necessary to divide the latter by 

 twice the former to obtain the N.A. equivalent of one millimetre 

 of chord. Thus a disc in my possession has a radius of 46*2 mm. 

 and a refractive index of 1*62. Consequently 1 mm. of chord is 

 equivalent to N.A. 0"0175. In any determination with this 

 disc, therefore, I simply adjust the indicators on the scale until they 

 are just seen simultaneously on opposite edges of the field, and 

 then multiply the distance between them by 0-0175 to get the 

 N.A. of the objective being tested. If these discs supplied by 

 Zeiss are made to a standard radius and refractive index, as 

 they probably are, there is no reason why the indicators should 

 not slide on a scale graduated to give the N.A. direct without 

 the necessity for multiplication by a constant. 



Joum. Quekctt Microscopical Club, Ser. 2, Vol. VIIL, No. 51, November 1902. 



