268 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



PAET II. 



EXPERIMENTAL WORK. 



The superiority of the optical surface of cyanin prisms, made 

 by pressing together the fused crystals between glass plates, and 

 the larger angles available lead one to suppose that more ac- 

 curate determinations of the dispersion can be made with such 

 prisms than with those obtained by the evaporation of alcoholic 

 solutions of the dye. Of special theoretical interest are the 

 values of the refractive indices for wave lengths within the ab- 

 sorption band. The results of the experiments give conclusive 

 evidence of the continuity of the dispersion curve, through the 

 absorption band in the yellow and show the presence of a second 

 absorption band in the ultra violet, beginning at wave length,. 

 368 fi fi, which has not been heretofore observed and which 

 should be taken into account in proving the Ketteler-Helmholtz 

 dispersion formula. 



The first part of the experimental work consisted of a syste- 

 matic examination of all the available aniline dyes known to 

 show anomalous dispersion with a view to determining : 



1st. How many of them could be formed into prisms by the 

 fusion method. 



2nd. The precautions necessary for the formation of prisms 

 of the best quality. 



From Fuchsin, Aniline Green, Hoffmann's Violet, Uranin, 

 and Aurine, prisms were made, but with difficulty. Their qual- 

 ity was, however, too poor to warrant any further work. The 

 Uranin absorbs moisture from the atmosphere and dissolves if 

 not protected. Acid Magenta, Erithrosene, Aniline Red, Rose 

 Bengale, Genetian Violet, and Methyl Blue could not be made 

 into prisms by this method. Gyanin fuses at about 116 °C, be- 

 coming a pasty substance for a few degrees and then a liquid of 



