Production of Circularly Polarized Light, 531 



of a FresnePs rhomb, while the length is not much different. 

 Hence in these respects the qualities of the new forms com- 

 pare favourably with those of a Fresnel rhomb, but in 



addition each has the advantage that the emergent beam of 

 circularly polarized light is always in the same straight line 

 as the incident beam whatever the orientation of the 

 polarizer. 



In the British Association Report for 1851, Prof. G. 

 Stokes described a new form of elliptic analyser consisting 

 of a plate of selenite which retarded waves of mean refrangi- 

 bility by about a quarter wave-length. This, mounted in the 

 manner described, was found to yield very accurate results. 

 but a little experience is needed in experimenting with it as 

 the tint produced perplexes the operator. It seems probable 

 that the second form of polarizer described above would 

 prove useful in the analysis of elliptic vibrations, as it could 

 be accurately set with respect to the analysing NicoFs prism, 

 an adjustment it is difficult to make with a quarter-wave 

 plate, for in this case the positions of the axes are not accu- 

 rately known. In the present form of analyser the edges of 

 the glass (these should be aceurately worked) forming the 

 aperture take the places of the principal directions of the 

 crystalline plate. Moreover, the tint would be absent. This 



