On the Anomalous Dispersion of Cyanin. 317 



3. The emission of cathode rays by the cathode due to the 

 influence of the ultra-violet light given out by the spark. 



In conclusion 1 wish to say that my heartiest thanks are 

 due to Prof. J. J. Thomson for the kind and liberal hospi- 

 tality w ith which he received me at his laboratory, and for 

 the continuous interest he has taken in my work. 



Cavendish Laboratory, Cambridge. 



XXIX. The Anomalous Dispersion of Cyanin. By Privat- 

 docent Dr. A. Pfluger, University of Bonn, Germany*. 



IN several papers Woodj has communicated a new method 

 of making prisms of solid cyanin, and also a repetition of 

 the measurements of the dispersion-curve, as I made and used 

 them several years ago for the proof of the Ketteler-Helmholtz 

 dispersion-formula \. 



Using my photographic method, Wood finds that cyanin 

 has a strong absorption-band in the ultra-violet, beginning at 

 the wave-length \=372 /jljj,. He says that in this part of the 

 spectrum it makes the measurements of the refractive indices 

 impossible, since the strong absorption prevents any impres- 

 sion on the photographic plate, even with a five-hours' 

 exposure. He continues § : " Pfluger found no traces of this 

 band, and gives values for the refractive index within its 

 limits. It seemed at first that the reason of this might be 

 found in the difference in the optical properties of fused 

 cyanin and that obtained by the evaporation of an alcoholic 

 solution, but we have found that films prepared in the same 

 way as those by Pfluger show the band also." 



My measurements of the refractive index [after the manner 

 accurately explained ||] in this part of the spectrum were made 

 on photographs, some taken with an exposure of 25 minutes, 

 others with 40 minutes. The plates show plainly the double 

 image of the iron lines used for the purpose of the measure- 

 ments. 



Furthermore, I have made photographs of the whole 

 absorption-spectrum, which show plainly the absorption-band 

 in the visible part of the spectrum, but not the faintest trace 

 of an absorption in the ultra-violet. In these experiments 

 the light of an iron spark passed through a quartz plate 



* Communicated by the Author. 



f Wood and Magimsson, Phil. Mag. [5] xlvi. pp. 380-386; [6] i. 

 pp. 36-45, January 1901. Wood, Phil. Mag. June 190] , pp. 624-627. 

 X Pflnger, Wied. Ann. hi. pp. 412-432; lxv. pp. 173-228. 

 $ Phil. Mag. Jan. 1901, p. 41. || Wied. Ann. lxv. p. 199. 



