Apparatus for production of Circularly Polarized Light. 517 



Summary. 

 1-5. The assumptions made by the Principle of Relativity 

 are stated and an attempt made to render some of them more 

 plausible at first sight. 



6. A difficulty connected with the composition of velocities 

 is examined and found to be due to verbal confusion. 



7. The confusions introduced by the word " real " are 

 discussed. 



8. The relation between dynamics and relativity is con- 

 sidered briefly. 



Leeds, November 1910. 



LVIII. On Apparatus for the Productionof Circularly Polarized 

 Light. By A. E. Oxley, M.Sc, Scholar of Trinity College, 

 Cambridge *. 



I^HE relative merits and defects of the quarter-wave 

 plate and Fresnel's rhomb as used in the production of 

 circularly polarized light are well known. For clearness it 

 may be well to state them here. When we are dealing with 

 monochromatic light, the quarter-wave plate has the great 

 advantage that as its axes are rotated the transmitted 

 circularly polarized beam is not displaced laterally as the 

 plate is rotated. Such a lateral motion is experienced with 

 a Fresnel's rhomb. On the other hand, when we are dealing 

 with white light, the use of a quarter-wave plate is impossible 

 since the phase-difference introduced between the com- 

 ponents transmitted, differs considerably for different wave- 

 lengths. In such a case the Fresnel rhomb must be used to 

 convert plane polarized light into circularly polarized light, 

 and vice versa. 



The present investigation was undertaken with the idea of 

 combining as far as possible the advantages of the Fresnel 

 rhomb and the quarter-wave plate in one piece of apparatus ; 

 the end being to produce an apparatus for which the 

 emergent beam should be in line with the incident beam, 

 and the various angles of incidence being so arranged as to 



produce a phase-difference of — for as large a range of wave- 

 length possible. 



Let the incident vibration upon a refracting medium be 

 represented by 



y = a.e mf ; 



* Communicated by the Author, having been read before the British 

 Association at the Meeting at Sheffield, 1910. 



