Interferometer Method of determining Phase Difference. 47 1 



The results agree sufficiently well to establish the exist- 

 ence of at least two periodic terms, but the differences 

 between the various solutions are so large that it is not easy to 

 say whether three or even a greater number may not be 

 involved. 



The somewhat unsatisfactory nature of this conclusion is 

 solely due to the circumstance that the measured ordinates 

 may be in error by an amount of the order of one-tenth of 

 the maximum amplitude of swing, and as analyses of arti- 

 ficially constructed functions show, small changes in the 

 values of the ordinates make unexpectedly large alterations 

 in the values of speed and other constants. 



Until means are found for obtaining a higher degree of 

 magnification of the movement, and, if possible, a diminution 

 in the breadth of the trace, it will be difficult to obtain more 

 definite results. 



University of London, 

 King's College. 

 Ajijr.1921. 



LII. An Interferometer Method of determining the Phase 

 Difference resulting from Metcdlic Reflexion, By H. P. 

 Waran, M.A., Government Scholar of the University of 

 Madras *. 



[Plate VIII.] 



WHILE investigating the possibility of a parallel plate 

 interferometer, by floating one transparent liquid as 

 water over another like mercury, the question arose as to the 

 difficulties that would arise from the difference in the 

 character of the reflexions from the two surfaces. In 

 the parallel plate of glass both the surfaces are identical 

 and the reflexions take place under identical conditions. 

 But in the present case nearly total reflexion at the top 

 water-air surface and simple reflexion at the bottom 

 mercury surface are utilised to get multiple reflexions from 

 the parallel plate of liquid. To see if this metallic reflexion 

 at the bottom surface was producing any disturbing factor 

 prejudicial to the success of this type of interferometer, a 

 control experiment was devised with a glass Lummer plate 

 which led to a very interesting observation. 



The glass Lummer plate (made by Hilger) was mounted on 

 a specially made ebonite mount, resembling very much the 



* Communicated by Prof. A. AY. Porter, F.R.S. 



