determining the position of the Plane of Polarization. 137 

 7=0, and the light is plane-polarized as before. The case we 



7T 



have to discuss, then, is a= -r* 



If a= j, and also for any particular value of \, (3 = j, then 



light of that colour is circularly-polarized. Hence, however we 

 alter 6, no change in the intensity of that light will take place. 

 If this circularly-polarized light comes from a prominent part 

 of the spectrum, it will be impossible to note small change in 

 the tint of passage due to the varying presence of other 

 colours. The difficulty experienced will be precisely similar 

 to that of fixing the position of the plane of polarization by 

 means of the yellow tint of passage instead of the violet tint. 



7T 



If ft is never so great as -~ t then, when both halves of the 



biquartz are of the same uniform tint, the position of the ana- 

 lyser determines the position of the ellipse ; but the uniform 

 tint will not be that due to excluding the yellow light of the 

 spectrum, but will contain lights of every colour, but not in 

 that proportion which constitutes white light. The tint may 

 be rosy or yellow. 



If a is neither nor -r, then co varies with X, and cannot 



4' 



possibly satisfy the solution for all values of 0. In this case, 

 then, both halves of the biquartz can never be made of the 

 same tint. As this is the general case, we conclude that the 

 biquartz is not a suitable instrument to use when, instead of 

 plane-polarized light, we have elliptically-polarized. 



The following table gives the values of to due to variations 

 in a and X when the light has passed through a quarter undu- 

 lation-plate of quartz. The values have been calculated from 

 Rudberg's table of indices, quoted on p. 317 of Glazebrook's 

 ' Optics.' The capitalletters refer to the lines of the spectrum. 







0. 



D. 



E. 



G. 





0. 



80°. 



90°. 



100°. 



126°. 



«=10°. 



u) 



2° 







-2° 



-6° 



*=20°. 



(j) 



4° 







-4° 



-13° 



«=44°. 



(!) 



39° 







-39° 



-43° 



The above is simply given as an illustration of the magni- 

 tude of the quantities involved in a particular case where it is 

 easy to make the calculations. I have tried the experiment 



