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XXXVII. The Anomalous Dispersion of Cyanin, 

 By R. W. Wood*. 



INVESTIGATIONS on the anomalous dispersion of the 

 aniline-dyes have been made, for the most part, by 

 means of solutions of these substances contained in prismatic 

 vessels, the dispersion of the solvent being eliminated by 

 means of a second fluid prism of the same angle, in contact 

 with and opposed to the first. Such prisms, when the solution 

 is at all concentrated, allow the passage of very little light, 

 and that close to the refracting edge. They are extremely 

 inconvenient to work with, and the phenomena are not very 

 easy to observe. Wernicke t made some determinations of 

 the refractive index of solid fuchsin, and Pfliiger % has lately 

 investigated a number of the dyes in the solid state. Both of 

 these investigations were made with prisms formed by allowing 

 an alcoholic solution to evaporate between a section of glass 

 tubing and a flat plate of glass. This method has great dis- 

 advantages : it is practically impossible to get a perfect optical 

 surface, and the prism-angle is so small that the deviation of 

 the ray is very slight. Pfliiger was unable to obtain prisms 

 with an angle greater than 2', and stated that only about one 

 in forty could be used at all, owing to the formation of rills and 

 streaks on the surface. Both of these methods seemed so 

 unsatisfactory that I have undertaken some experiments with 

 a view of preparing solid prisms of fairly large angle and 

 with perfect optical surfaces, that would allow of a more 

 accurate determination of the dispersion-curve. 



I first tried to find some solvent for the aniline capable of 

 solidifying, and had considerable success with Canada balsam, 

 and though this method was finally given up for a better one, 

 it seems worth while to outline some of the results obtained, 

 as they proved interesting in another way. 



Filtered Canada balsam was boiled down until a drop placed 

 on a slip of glass solidified on cooling. The aniline was then 

 added and a little of the hot solution daubed on a strip of hot 

 plate-glass. A similar strip, well heated, was lowered onto 

 this and pressed down into close contact along one edge, the 

 other being held up. squeezing the fluid into a thin prism of 

 two or three degrees. A prism of clear balsam was then 

 formed on the back of the glass strip to neutralize the effect 

 of the solvent, as is done in the case of fluid prisms. The 

 whole formed a solid rectangular piece, appearing thus in 



* Communicated by the Author. 

 t Pogg. Ann. vol. civ. p. 93. 

 % "Wied. Ann. vol. lvi. p. 412. 



