o86 On the Anomalous Dispersion of Cyanin. 



part of the curve. Pfliiger's work has shown that the curve is 

 undoubtedly continuous, and prisms prepared by his method 

 appear to be the only ones suited for investigations in this 

 portion of the spectrum ; for I have not yet succeeded, even 

 with very strong pressure in a vice, in squeezing melted 

 cyanin into films thin enough to transmit yellow light. 



It is difficult to get accurate readings close to the absorption- 

 band, since the only part of the prism which allows the passage 

 of rays in this part of the spectrum is a strip only a small 

 fraction of a millim. wide along the refracting edge, the prism 

 acting as a narrow aperture, and giving Fraunhofer diffraction 

 phenomena of the first class. Instead of a sharp image of the 

 slit we have a broad diffused band which cannot be accurately 

 set on the cross-hairs of the eyepiece. I have accordingly 

 indicated by question-marks those values on the curve which 

 I regard as not accurately determined. Comparison of the 

 dispersion-curve figured, above with the one given by Pfliiger 

 in his last paper* shows very close agreement on the red side 

 of the absorption-band, while on the blue side his curve runs 

 a little higher than mine. It is of course very probable 

 that the optical properties of cyanin solidified from fusion are 

 not the same as as when crystallized from alcoholic solution. 

 The change in the surface-colour indicates that this is so ; 

 consequently prisms prepared by the two different methods 

 could hardly be expected to yield identical results. The 

 surface-colour, moreover, changes on exposure to the air, 

 being an orange-yellow when the glass plate is first removed 

 and changing to a plum-colour in the course of a day or two. 



A determination of the extinction-curve of cyanin films 

 prepared from fused crystals will of course be necessary before 

 anything definite regarding the difference in the optical pro- 

 perties can be said. Facilities for such an investigation are 

 not at my disposal at the present moment ; but 1 hope in the 

 near future to make a complete study of the optical constants 

 in their relation to the Ketteler-Helmholtz dispersion formula, 

 as has been done by Pfliiger. The superiority and large angle 

 of the prisms prepared from the fused dye make it seem worth 

 while to repeat Pfliiger's work on this particular substance, 

 particularly as he found some discrepancies in the extreme 

 violet. 



Physical Laboratory of the 

 University of Wisconsin. 

 Madison, June 1st, 1898. 



* Wied. Ann. vol. lxv. p. 173 (189b). 



