3o 



L. B. Tuckerimm 



The formal difference between Q 3 and Q' 3 lies wholly in the dif- 

 ference of thickness of the two halves of the halfshade. No gen- 

 erality is lost by referring the incident light to its major and 

 minor axes. Then from the general equations: (15) and (16) 



P=P 



Q =P t 



°i+e\ 



K 



S~P 



2e 



and 



Q S =P 



°i+e\ 



Z.e. 



•M-d 



+ COS2(0, i) COS 2 (l,2) COS 2(2,3) 



— cos2(o, 1) sin2 (1, 2) sin2(2,3) cos27rA^ 2 



— sin2(o, 1) sin2 (1,2) cos2(2,3) cos27rA r j 



— sin2(0, I ) COS2(l,2) sin2 (2,3) COS27rA 7 1 COS27rA^ 



-j-sin2(o, 1) sin2(2, 3) 5^2^ sin27rA r 2 



(53) 

 -)-sin2(i, 2) cos 2 (2, 3) %m.2TrN^ 



+ COS 2(l,2) sin 2(2,3) sm 2tvVj COS27rA r 2 



+ sin 2 (2, 3) cos 2ttN x sin 2^^ 



A. »-»■ Elliptic Halfshade s-^ Compensator s-*- Nicol 



For this arrangement the symbols have the following signifi- 

 cance : 



Order of the one half of halfshade =■ A^ 

 Order of other half of halfshade =N\ 

 Order of compensator — ^ 



Angle between major axis of incident light and principal azimuth 



of halfshade = 2^(0,1) 

 Angle between halfshade and compensator = 2^(1,2) 

 Angle between compensator and nicol = 2^(2,3) 



This arrangement is useful in measuring small ellipticities with 

 a compensator of low order, since it enables the halfshade to be 

 conveniently placed relatively farther from the observing tele- 

 scope and is easily applied to the ordinary polariscope. 



186 



