354 Miss H. G. Klaassen on Change of Phase on Reflexion 



§ 6. Change of Phase on Reflexion : Air— Fuchsine. 



I have observed roughly the change of phase which occurs 

 on reflexion from air to fuchsine and to other dyes in the 

 following way : — 



Press a lens on to a layer of dye and examine in mono- 

 chromatic light. The layer of dye must be deposited so as 

 to have a very level surface, shown by its bright reflecting 

 power. It must also be so thick that the light coming from 

 the lower surface is negligible. In consequence of the 

 change of phase on reflexion, air-dye, we do not get a black 

 centre to Newton's rings. 



By examination of the central spot in light of different 

 colours we can make an estimate of the change of phase. 

 The following substances were examined, and each showed a 

 maximum abnormal change of phase in the coloured light 

 which is most absorbed by the dye, — fuchsine, methyl aniline 

 violet, acid green, eosin. 



This effect is most marked in methyl aniline violet, where 

 the central spot is very bright in yellow and green light. The 

 direction of the change is an increased retardation where the 

 absorption is great. 



If n 2 -\-g 2 for fuchsine were greater than 1 for all rays, 

 these observations would be in qualitative agreement with the 

 formula 



tan o = -5- — f ? ; 



n 2 + g'—n 2 ' 



both exhibit a maximum retardation in the rays for which the 

 absorption is greatest, falling off to it at either end of the 

 spectrum. 



Pfl tiger has shown that for light between F and g the 

 index of refraction in fuchsine < 1. 



If we view the rings produced by pressing a lens on to 

 fuchsine in blue light we might expect to find a bright centre. 



On the contrary, the centre was dark in blue light and, as 

 as far as I could tell, the change of phase differed very little 

 from ir. 



§7. Colours of thin Plates of Fuchsine. Table A. and 

 Table B. 



A change of phase differing from or it has been observed 

 on reflexion at the surface of fuchsine, the amount of this 

 change depending on the index of refraction of the adjacent 



