﻿950 Prof. C. Baras on the 



From these results the value of the differential coefficient 



dfi = _2B 

 d\ X 3 



may also be obtained for any wave-length \. This and the 

 corresponding value * of 



needed in § 9, in the reduction of the observations of the 

 earlier paper | will be computed by accepting the mean 

 value Bxl0 10 = '48. 



9. Computation of the Shift of Centres in Elliptic Interferon 

 metry. — In the preceding report % I showed that the path dif- 

 ference, y, corresponding to the centres of ellipses throughout 

 the spectrum is 



3/ = 



cos R 



H£> n 



where e is the thickness of the ruled glass plate and fi its 

 index of refraction, for any given colour of wave-length X. 

 11 is the corresponding angle of refraction for light incident 

 at an angle I, or sin 1 = //, sin R. 



In the former experiment 1 =45°. In view of the oblique 

 position of the grating, however, the micrometer motion N 

 of the opaque mirror introduces an additional displacement. 

 Let N be the coordinate for this mirror. Then 



N=#— ^sinEtanE (2) 



Hence the readings of the micrometer are determined by 

 (subscripts c referring to centres of ellipses), 



N c =-^_ L C os 2 E-X^ (3) 



c cosEy dV 



Taking the E line as a standard of reference, X c and the 

 quantities here in question were now computed both for light 

 crown glass (10 10 B = "456, in accordance with the data for p 

 and X given in Kohlrausch's tables) and for the glass of the 

 grating actually used (10 10 B = "48), as measured in para- 

 graph 8. Ay c and AN C refer to distances from the value 

 corresponding to the E line. N c , ?/ c , and X are to be given in 

 centimetres, and the angle of incidence is 45°. The data 

 determine the full lines in fig. 6. 



* The symbol fi for index of refraction is resumed. 



t Carnegie Publications, No. 149, p. 69 (1911). 



J Carnegie Publications. > T o, 149, 44, p. 66 e-tseq. (1911). 



