On the Anomalous Dispersion of Cyanin. 381 



cross section. These prisms worked fairly well, and had a 

 great advantage over the fluid ones in that they were per- 

 manent, compact, and could be used either 

 vertically or horizontally. The rectangular 

 trough with the diagonal partition, devised 

 by Soret, cannot be used for showing the curved spectra by 

 the method of crossed prisms, unless one has a telescope 

 moving on a vertical circle. Balsam prisms coloured with 

 different dyes are especially advantageous for exhibiting 

 dichrornatism, and it is on this account principally that I have 

 brought them in. For showing this, however, it is best to 

 dispense with the second prism of clear balsam, as the sepa- 

 ration of the transmitted colours by the action of the prism 

 is an advantage. If we look at a brightly illuminated slit or 

 a candle-flame through the thin edge of a prism stained with 

 " Brilliant Green," we see a very bright green image, and 

 close to it a faint red one. On moving the prism along so as 

 to increase the thickness of the transmitting layer, the green 

 image fades very rapidly, leaving the red almost unaffected, 

 and finally the red image is all that remains. This separation of 

 the transmitted portions into two images renders the mecha- 

 nism of dichrornatism much clearer than the usual method of 

 showing it by the superposition of a number of flat plates of 

 coloured glass. 



The balsam double prisms for anomalous dispersion were 

 not quite what 1 wanted, as they were in reality nothing but 

 solutions ; and though the dispersion was much greater than 

 anything I had obtained with alcoholic solutions, I sought a 

 better method. To fuse the dye and press it out into a thin 

 wedge between two pieces of plate-glass appeared to be the 

 best line to work on, and I made a trial with cyanin. This 

 proved to be a lucky choice, as it is the only suitable dye that 

 I have found thus far, all the others decomposing at or below 

 the fusing-point. But with cyanin 1 succeeded after one or 

 two trials in preparing some admirable prisms, which yielded 

 results far ahead of anything that I could produce by any 

 of the other methods. From a piece of plate-glass 5 to 

 7 millim. in thickness a number of rectangular pieces are cut 

 measuring about 4 centim. square. A pair of these plates are 

 carefully cleaned and a narrow strip of paper pasted along 

 and close to the edge of one. Along the opposite edge is 

 strewn a little train of cyanin crystals about 2 millim. wide. 

 The train should be made of uniform depth, and pushed into 

 a straight line with another piece of glass. Both plates are 

 now laid on a sheet of asbestos-board over a bunsen-burner, 

 and heated until the cyanin fuses. Just before the fusing- 



Phil. Mag. S. 5. Vol. 46. No. 281. Oct. 1898. 2 E 



