24 Mr. R. A. Houstoun on Total Reflexion at the 



Attempts were then made to observe the effect of the 

 collodion film in ordinary reflexion and refraction. In the 

 first case the one component was too weak at the polarizing 

 angle to give good results. The phase-difference produced by 

 a surface-film in refraction is given by 



tan A d = — e sin (f> tan (<£ — (f>')* } 



where cf> is the angle of incidence and cf)' the angle of 

 refraction. The following measurements were obtained: — 



E. 



0. 



Ad (air to glass). 



Ad (glass t( 



o 1 



o 





o 



o 



154 



30 





0-7 



-1-3 



154 12 



38 





0-6 



-0-3 



153 22 



46 





0-8 



-o-o 



151 32 



54 





0-7 



-1-1 



Owing to some unknown circumstance the measurements 

 from glass to air are not so accurate as those from air to glass. 

 The theoretical value, obtained by using the value of e already 

 found from observations on total reflexion, is +0°*6. 



III. Total Reflexion at the Second Surface of a Thin Plane 

 Parallel Plate. By Robekt A. Houstoun, M.A., B.Sc, 



Glasgow University 1851 Exhibition Scholar f. 



THE method used in the experiments described in the 

 preceding paper was also employed to investigate total 

 reflexion at the second surface of a thin plane parallel plate. 

 A prism, the same as the prism used in the collodion experi- 

 ment, was coated by Bottger's method with a thin homo- 

 geneous layer of silver on its hypotenuse surface. Sodium 

 light, plane-polarized at an angle of 45° to the plane of 

 incidence, entered the prism, was reflected by the silver layer 

 on the hypotenuse, and the relative phase- difference produced 

 between the components, parallel and perpendicular to the 

 plane of incidence, was measured with the Babinet compen- 

 sator and analysing nicol. The angle of incidence on the 

 hypotenuse surface was usually such that the light would 

 have been totally reflected, if there had been no silver film 

 there. The silver film was then changed into silver iodide, 

 and similar measurements were made. 



Theoretically this is the case of the plane parallel plate, 

 the thickness of which is small in comparison with the breadth 

 of the incident light-wave, for the case that the three media 



* P. Drude, Wied. Ann. vol. xliii. p. 145^(1891). 

 t Communicated by Professor A. Gray, F.R.S. 



