Fringes for Internal Reflexion. 505 



the ray — opposite to the side on which it appeared just 

 before the principal incidence, for otherwise when we reach 

 normal incidence the two vectors P and Q would be on the 

 same side of the normal, as in fig. 2. But in the case of a 



Fis-. 2. 





1 \ 



5 I 



R 



cj 1 



k 





* 





s 1 



\ ^ 



^ \ 



K 



^ \ 



pencil polarized in the other way, the two vectors appear on 

 opposite sides of the normal ; and in the case of normal 

 incidence the plane of polarization is of course immaterial 

 for ordinary reflexion. 



The complete results follow from the Fresnel reflexion 

 formulae ; from these it appears that the amplitude does 

 change sign as we pass through the principal incidence. The 

 Jamin experiments are in accordance with the Fresnel 

 formulae, and the phenomena of the elliptic polarization near 

 the principal incidence are in agreement with the Cauchy 

 extension of the Fresnel formulae, which take into account a 

 transition layer at the surface. It is hardly necessary to 

 remark that these formulae follow as results of the electro- 

 magnetic theory. 



There is, however, as yet no evidence for the actual phase 

 changes which reflected light undergoes when the reflexion 

 is internal. There is evidence for the difference of phase 

 changes between light polarized in and perpendicularly to 

 the plane of incidence. Such instruments as the Fresnel 

 Rhomb depend on this difference, but as to the actual value 

 in either case as vet there is no evidence. The Stokes 



