830 



Prof. Carl Bar us on the 



need never be difficulties due to elliptical polarization, for 

 the vertical and horizontal vibrations pass unchanged. If, 

 however, rotary polarization occurs, the case of the oblique 

 path is more complicated, but it does not interfere with the 

 measurement. 



The present method inherently presupposes a knowledge 

 of the dispersion of the crystal. Whenever this term is of 

 the nature of a correction, the coefficient in the simplified 

 Cauchy equation of two terms may, at the outset, be inserted. 

 For accurate work Cauchy's equation as far as the fourth 

 power of wave-length must be used. It will be shown more- 

 over, that the method is self-contained, by affording means 

 for determining the coefficients of the equation in question. 

 Initially no more than a measurement of the refraction of 

 the extraordinary ray in terms of the ordinary ray will be 

 attempted. The dispersion constant found for these rays 

 from data for two Fraunhofer lines may then be used for all 

 wave-lengths lying between. 



2. Equations. — If e' , I, R 3 fx ', denote the thickness, the 

 angles of incidence and refraction, and the index of refrac- 

 tion of the grating, and e, i, r, fi, the corresponding quantities 

 for the plane parallel plate inserted in one of the component 

 beams of the interferometer; if, furthermore, V and b denote 

 the corresponding coefficients in the simplified dispersion 

 equation of Cauchy /jL = a + b/X 2 , so that 



~\d/jL/d\ = 2b/\ 2 , 



the coordinates of the micrometer in the absence of the plate 

 will be, if /ji a is the absolute index of refraction of air, 



N -''"'( C ° SR+ AV«) + *- ' • « 

 where e/j a is the air-path coextensive with the thickness of 



Fig-. 2. 

 e 



^_ \^0J 



^r\cA 





l\ 



lUT 



i^ J 





dC^ 





the plate in its normal position AA (fig, 2), I being the 

 incident beam of light. If the plate is inserted at an angle 



