250 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



two given wave lengths may vary considerably. At about this 

 time Fox Talbot, while working with the double oxalate of chro- 

 mium and potash, made the remarkable discovery that prismatic 

 crystals of this substance have the power of deviating the red 

 more than the blue. Nothing was published on the subject, 

 however, until nearly twenty years later, when Le Eoux 1 an- 

 nounced the fact that by a hollow glass prism filled with iodine 

 vapors the red rays are bent more than the blue. Talbot 2 at 

 once recalled his early experiments and published an account 

 of what he had done. Speaking of the arrangement of the col- 

 ors in the spectrum produced by the double oxalate of chromium 

 and potash, he says : "The colors were very anomalous, -and 

 after making many experiments I came to the conclusion that 

 they could only be explained by the supposition that the spec- 

 trum, after proceeding for a certain distance, stopped short and 

 returned upon itself." 



M. Hurion 3 made quantitative measurements upon iodine va- 

 pors and found the refractive index for the violet and red rays 

 at TOO C to be ^==1.019; ,1=1.0205. 



The first real impulse for extensive work upon this phenom- 

 enon was given by Christiansen 4 in 1870, when he discovered 

 anomalous dispersion in an alcoholic solution of fuchsin. 

 Christiansen noticed that in the transmitted beam the green 

 light was absent, that the red, orange, and yellow rays were re- 

 fracted in their usual order, but that the violet was deviated less 

 than the red, so that a dark band lay between it and the red. 



Fig. 1. 



The relative arrangement of the colors in this spectrum is 

 shown in figure 1, in which the approximate intensities of the 



i Le Roux, Comp. Rend., LV., p. 126 (1862). Pogg. Ann. CXVIL, p. 659; Ann. de Chim. 

 et de Phys. (3) LXL, p. 285 (1861). 

 1 Talbot, Proc. Roy. Soc. Edin. 1870-1871. 

 » M. Hurion, Jour, de Phys. (1), VII., p. 181. 

 * Chrietianien, Pogg. Ann. CXLL, p. 479; CXLIIL, p. 250; CXLVL, p. 154. 



