Reflexion at the Surface of Highly-absorbing Media. 349 



electrostatic strain. Quincke* has observed the effect in the 

 case of carbon bisulphide. The relative retardation which 

 he obtained for the two components in an electrostatic field of 

 one C.G.S. unit was 3"2 x 10 -7 A. per centimetre. In this 

 case the effect is proportional to the square of the intensity of 

 the field. If we assume the same law for a magnetic field 

 and reduce the value 5*5 x 10 -5 X — the limit of sensibility for 

 198 cm. and 2600 C.G.S. units— we have 4-1 x 10~ 14 X as the 

 relative retardation per centimetre in one C.G.S. unit field 

 which could have been observed. 



In the one case we have used the electrostatic unit, and in 

 the other the electromagnetic unit, since we are comparing 

 dynamic effects and must refer them to the dynamic units in 

 each case. 



We may also conclude, according to Maxwell's view of the 

 state of polarization and stress in such a medium, that the 

 pressure at right angles to, and the tension along, the lines of 



force I equal in both cases to ^- ) affect the propagation of light 



by an amount less than 2 , 0xl0~ 14 A, for a C.Gr.S. unit of 

 intensity per centimetre. 



Physical Laboratory, University of Nebraska, 

 Lincoln, Neb., U.S.A., June 19, 1897. 



XLIV. Change of Phase on Reflexion at the Surface of Highly - 

 absorbing Media. By Helen G. Klaassen, Associate and 

 Lecturer, Neionham College f. 



§ 1. TF a layer of aniline dye is deposited upon glass by 

 J_ dropping a little of the dye dissolved in alcohol 

 upon the glass and allowing the alcohol to evaporate, just at 

 the edge of the layer so deposited we often see a film, which 

 in reflected light is of quite a different colour from the rest of 

 the dye. If, for instance, we deposit fuchsine, which has a 

 green surface-colour, we often seen a straw-coloured film 

 round the edge. The contrast in colour is more marked if 

 instead of looking directly at the fuchsine we turn over the 

 glass and look at it from the other side. The light reflected 

 at the surface of separation of glass and fuchsine is bluish 

 green, while the film round the edge is red or orange. 



I attempted to make larger patches of this straw-coloured 

 film, and succeeded in obtaining thin films of continuous 



* Wied. Ann. xix. p. 729 (1883). 

 t Communicated by tbe Author. 



