at the Surface of Highly-absorbing Media. 353 



film as seen from either side. If we may judge from such 

 very rough observations that this is the case, it seems that 

 a thin plate of fuchsine placed between two transparent 

 media of equal refrangibility is free from the peculiarities of 

 Table B. 



The change of phase on reflexion absorbent-transparent 

 = that on reflexion transparent-absorbent +tt approx. 



Theoretically this relation should hold if the change of 

 phase on refraction is small compared with that on reflexion. 



§ 5. The Change of Phase on Reflexion: Glass— Fuchsine. 



Wernicke * and Potier f have exhibited the change of 

 phase which occurs on reflexion from glass to fuchsine. 

 Potier \ gives the following measurements : — 



8 = retardation on reflexion from crown-glass to fuchsine. 



8_ D. E. b. F. g. 



~2tt -41 -30 -25 -18 



Wernicke's § results are not in exact agreement, but he 



also finds a gradual increase of acceleration as we pass from 



the blue to the red end of the spectrum, starting from nearly 



at the blue end and amounting to it at the red end. 



Potier made similar measurements, using a flint-glass of 



high index, 1*96, which he says is higher than the index for 



any ray in fuchsine. He finds a maximum acceleration 



8 

 u— ='34 in the green between D and E, falling off to at 



either end of the spectrum. Potier points out that his 

 observations are a qualitative confirmation of the theoretical 

 formula 



tan 8 — 



%n g 



n l + g l -n^ ' 



w = refractive index of transparent medium, 

 n = refractive index of absorbing medium, 



a = coefficient of absorption x -. — • 



r 4-7T 



* Pogg. Ann. clix. p. 198. 

 t Comptes JRendus, cviii. p. 995. 



\ I have the permission of Prof. Potier for inserting the minus sign before 

 8 



§ See second note on page 352. Wernicke's measurement of refractive 

 index. 



Pniiger {he. tit.) gives a much higher maximum index. Sirk finds the 

 value 2 - 44 for 0., using melted fuchsine. 



Pfliiger attributes the disagreement between his and Wernicke's mea- 

 surements to the fact that Wernicke's fuchsine was not free from alcohol. 



