REVERSED AND NON-REVERSED SPECTRA. 



129 



If x is the displacement in centimeters in the ocular, 5 = ^/19.5, the denom- 

 inator being the length of the telescope. The successive values of x in terms 

 of N, the mirror displacement, are given graphically in figure 87 for the dif- 

 ferent values of E' = E-{-e/co$ R in question, and from them dd/dN c was 

 taken graphically. All the curves are appreciably straight, except the one 

 for '=1.3 cm., which is definitely curved. 



4 



The values of dd/dN c observed and computed obviously agree as closely 

 as may be expected for mean conditions of the spectrum between green and 

 red and of the mode of procedure. The small coefficient for carbon bisul- 

 phide is, moreover, in keeping with its large value of B or high dispersion. 

 In a sensitive displacement interferometer B like E' D should all be small ; 

 but unfortunately this implies large ellipses. The beautiful small ellipses 

 obtained with the carbon-bisulphide plate are of little avail because of their 

 sluggish motion. In fact, the small thickness of the carbon bisulphide layer 

 and the reduced coefficient observed are quite striking in comparison with 



^lass. Moreover, since for an order n 



o 



dd i 



X 2 



dn 2 D cos <J (cos R + zB/"\ z cos R) N 



the effect of a large B, though it reduces the general size of ellipses, does so 

 but slightly, since the term in B can not be more than a few per cent (3 to 6) 

 of the other term in the binomial. 



