the Echelon Spectroscope. 387 



obtained for this glass was as follows:— ..;...;. 



1 kkkg 8-03 xlO 5 1-53 xlO 12 



where X is expressed in 10" 10 metres. . - ■ 



From this equation interpolated values of the refractive 



index for other wave-lengths were calculated, and a curve 



plotted, as shown in tig. 3 (p. 389). 



The value of dji/dX, which it is also necessary to know for 



different wave-lengths, was obtained from the equation 



■ dfjL x 'd\ ■ 



16-06xl0 5 6-12 xlO 12 



where, as before, X is expressed in 10 -10 metres. 



A curve for this was also plotted, and is shown in fig. 3. 



The thickness of the echelon plates was carefully measured 

 by means of a measuring micrometer reading to 1/10,000 of 

 an inch, and was found to be 7*424 + *006 mm. ■ this accuracy 

 being sufficient for all measurements made with the instrument. 



We may now consider the expressions obtained for the dis- 

 persion of the instrument *. 



Fig. 2. 

 i i 



i 



L 



K 



3 



be the order of the spectrum of the light observed, r 

 angle of deviation from the normal, 

 wave-length of the light, 



refractive index of the glass for the wave- 

 length considered, ". J 

 thickness of the echelon-plates, 

 width of a step : 

 retardation produced by each step of the echelon 



. . . (i) 



Urn 



be i 



e 



?j 



\ 



J? 



:M. 



5? 



t 



V 



.<? 



>> 



the 

 (fig- 2) 



Bm\ = fit — t cos 6 + s. sin 6. . . 



Approximately raA-=(/u — 1)£, since is very small 



... * Mrchelaou,;Astroph.ya^ Jouriu foci ct't. a- 



