214 Mr. W. B. Cartmel on the Anomalous 



have not yet been fully determined except by indirect methods. 

 Very complete absorption-curves have indeed been given for 

 solutions of different concentration, but not for solid fuchsin. 



It was therefore thouoht that it mio^ht be of interest to 

 determine the absorption and dispersion directly, both of 

 these upon the same identical fuchsin, and as a very good 

 determination of the dispersion-curve has already been given 

 by Pfliiger *, who measured the deviation produced by a thin 

 wedge of solid fuchsin, it was decided to redetermine this, 

 using interferential means. This would present the advan- 

 tage of a redetermination by a different method, and, further- 

 more, the measurements of absorption and dispersion could 

 both be made upon the same film. 



Films were therefore prepared in the usual way by dipping 

 glass plates into an alcoholic solution of fuchsin and allowing 

 the alcohol to evaporate. The fuchsin upon which the first 

 experiments were made was some that had been purchased 

 for general laboratory purposes, and it was found that the 

 dispersion and absorption had values very much lower than 

 those given by Pfliiger. Some fuchsin of the same kind as 

 that which Pfliiger had used was therefore imported from 

 Kahlbaum in Berlin, and upon this were carried out the ex- 

 periments which form the basis of the following paper. 



As already pointed out by Woodf, the great difficulty in 

 determining the dispersion of strongly absorbing substances 

 by interferential methods is that the ray which passes through 

 the substance is so reduced in intensity that it is not capable 

 of causinp- interference when it meets the undiminished lioht 

 of the other ray. This may be easily conceived to be the 

 case with fuchsin, when we consider that a film of fuchsin a 

 wave-length thick transmits only five parts in one hundred 

 million of the incident light within the absorption-band. 

 Experiments were therefore made Avith the object of reducing" 

 if possible the intensity of the light in one of the paths of the 

 interferometer without producing any change in its optical 

 length, in order that the fuchsin might be placed in the more 

 intense beam. However, none of the various plans tried 

 were adopted. It was then decided to use a form of inter- 

 ferometer in which the light does not return upon itself, and 

 for two reasons. First, because in traversing the film twice 

 the diminution in intensity is squared while the retardation 

 is only doubled ; and, second, because the enormous reflexion 

 from the surface of the film obscures the fringes in the 

 ordinary form of interferometer, but in the type used the re- 

 flected light does not reach the observer's eye at all. 

 * AVied. Ann. xlv. p. 203 (1898). f Phil. Mag. i. p. 43 (1901). 



