tillyer: variation of equivalent focus 



483 



The metal plate used for isolating the individual rays has been 

 calibrated, and the height h determined ; the angle u can be obtained 

 from 



a + h a •. 



tan u = — r— = -> 



d 



V 



and consequently E and AE — A v. This set of measures can be 

 repeated for different wave lengths, and the variations in the 

 quantities determined; from A ?; we find the axial chromatic aber- 

 ration and the departure from the Gauss condition, and from 

 AE — Av, the oblique chromatic aberration and the variation of 

 coma with color. 



0.01 n: 



0.01 E 





The diaphragm plate, generally used with moderate size lenses, 

 has holes of 1 mm. diameter arranged in a line and located at about 

 3 mm. intervals. The error in the known position of the- holes is 

 less than 0.005 mm. which corresponds to an error of about one 

 part in ten tliousand in A E for the edge ray of an ordinary 

 small lens. The shadowgraphs are easily measured on a small 

 Zeiss comparator with sufficient accuracy for rays somewhat 



