14 Mr. R. A. Houstoun on the Effect of a 



set exactly. This second cause of error was investigated theo- 

 retically, and does not seem to be an appreciable cause of error 

 in the observations made. The first cause of error, the effect 

 of inner reflexion, has been discussed by Prof. W. Voigt *. 

 In the instrument used, the rays reflected on the surfaces C 

 and D form separate images in the field of vision, and the 

 intensity of the light reflected at B and again at A is too 

 small to be taken into consideration. There remains only the 

 light reflected on the surfaces E and A, the intensity of which 

 is perhaps j^-$ of the intensity of the light that comes direct 

 through. As the distance between the surfaces A and E is 

 great, and the light is not absolutely homogeneous, this error 

 does not come into consideration. 



In the elementary theory it is assumed that light is propa- 

 gated in quartz in two waves, plane-polarized at right angles 

 to one another. Prof. Voigt has, however, shown in a recent 

 article f, that it is propagated in two elliptically polarized 

 vibrations, the ellipses being long and thin, having their long 

 axes perpendicular to one another, and being traversed in 

 opposite directions. The effect of the ellipticity of the 

 vibrations on observations made with the compensator was 

 investigated, and a result in accordance with Prof. Voigt's 

 theory obtained. The investigation is given here. 



For simplicity we can consider the compensator as con- 

 sisting of tw r o plane parallel plates of quartz, with their axes 

 Hj and H 2 exactly at right angles to one another. Let us 



rJj 



suppose that the incident light consists of two vibrations, 

 E sin T along H b and E' sin T' along H 2 . To take account 

 of the elliptic propagation in the quartz we must replace 

 these two linear vibrations by four elliptic vibrations, a right- 

 handed and a left-handed vibration having their long axes in 



* W Voigt, Wied. Ann. vol. xxii. p. 236 (1884). 

 t W. Voigt, Gott. Nach. 1903, p. 155. 



