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



than the surface-colour of fuchsine in air, but a comparison of 

 the two columns of Table A shows that this is not the com- 

 plete explanation of their difference. 



From Table B we see that the maxima and minima in 

 blue light do not occur at the same thickness of film as seen 

 directly or through the glass. In a transparent film there is 

 a maximum or minimum at zero thickness. This is not the 

 case with fuchsine films observed in yellow, green, or blue 

 light. The peculiarities of Table B can be expressed by 

 saying that the interference-bands as seen from the glass side 

 are displaced towards the thicker end of the film, the dis- 

 placement increasing as we go from the red to the blue end 

 of the spectrum. The second column shows the displacement 

 of the first minimum in yellow and green light and of the 

 first minimum and maximum in blue light. 



If the film is viewed directly the bands in blue light are 

 displaced in the opposite direction. As the displacement is 

 towards the thin end of the film the first minima disappear. 



Table B shows a very large wave-length for blue light in 

 fuchsine compared with that for red. 



This is in agreement with the recent measurements of 

 PflUger*", who finds a much larger anomalous dispersion 

 than Wernicke f- 



This is no doubt due to the fact that my films, like those 

 of Pfliiger, are very free from alcohol. Some films which 

 I have kept for two years show no sign of change. 



§ 4. The suggestion might be made that the maxima and 

 minima observed in these highly absorbing films are not mainly 

 due to the reflexion at the back surface. To test this I 

 smeared some oil on the film and viewed it from the back of 

 the glass on which the fuchsine is deposited. In the very 

 thin films there is no longer a diminution of illumination with 

 increase of thickness. The minima of Table B and the 

 peculiar colouring of Table A, 2nd column, thickness II. 

 disappear. The maxima and minima in blue light are also 

 displaced towards the thin end of the film. If we turn over 

 the glass and view the film through the oil, the maxima and 

 minima are also displaced, but in the opposite direction. The 

 extent of the displacement is such as to make the maxima 

 and minima occur at approximately the same thickness of 



* Wied. Ann. lvi. p. 424. 



Refractive Index. 



t A 1-73 



B 1-81 



C 190 



9 1-31 



H 1-54 



