Anomalous Dispersion of Carbon. 407 



particles in smoke-films is less than 0'00026 mm., while the 

 regular reflexion at normal incidence of violet light by the 

 deposited films indicates that the diameter of the particles 

 must be less than one-eighth of the wave-length of violet 

 light. (Assuming £ \ to be the maximum allowable phase- 

 discrepancy.) 



The dispersion was first measured with a Michelson inter- 

 ferometer, illuminated with monochromatic light of various 

 colours obtained by prismatic analysis. The fringes were 

 photographed and measured as described in the paper on 

 Cyanine*, readings being obtained between wave-lengths 

 '00040 and *000(5G. The results showed a staady increase of 

 refractive index in passing from blue to red. The results 

 obtained with the deposited films were less satisfactory, owing 

 to the poor quality of glass. They were prepared in the 

 following way : — An incandescent lamp bulb was cut in two 

 and a small piece of German plate-glass introduced; the two 

 halves of the buib were then sealed together in the blowpipe 

 flame, and the air exhausted, after which the lamp was run 

 for several hours at considerably above its rated candle- 

 power. Very uniform metallic-like films were obtained in 

 this way, which gave elliptic polarization. The poorness of 

 the glass, however, made accurate measurements impossible, 

 owing to the irregular curvature of the fringes, and the 

 work will be repeated with films deposited on plane-parallel 

 optical glass. This seems to be important, since it seems 

 oolite possible that the films may be molecular in structure ; 

 the particles are certainly much smaller than in the case 

 of the smoke-films, and may give a different dispersion 

 curve. As far as it was possible to judge the deposited films 

 showed the same anomaly as the smoke-films. 



No attempt was made to determine the absolute refractive 

 index, since accurate determinations of the thickness of the 

 smoke-films are difficult. Even were this possible the values 

 obtained would not mean much. Relative values only were 

 obtained, as indicated in the following table, where n 

 represents the fringe displacement, measured in fringe-widths, 

 X the wave-length, and n\ the product which is proportional 

 to the refractive index, as shown by the usual formula 



{p—l)e=n\ 3 



where e is the thickness of the film. 



* " The Anamalous Dispersion of Cyanine," R. W. Wood and O, 

 E. Magnusson, Phil Mag. Aug. 1900. 



