the Anomalous Dispersion of Cyanin. 



39 



These results are shown graphically in fig. 1 (p. 40), and show- 

 conclusively that the dispersion-curve is continuous through 

 the absorption-band. 



Measurements in the ultra-violet were made by photo- 

 graphy. A Rowland concave grating was used, the slit, 

 grating, and photographic plate being on the circumference 

 o£ a circle in the customary manner. The slit was illumi- 

 nated with sunlight in some of the experiments, and by the 

 light of an arc formed between an iron rod and a revolving- 

 iron disk in others. A cyanin prism, formed on a quartz 

 plate, with an angle of 2' 51' /# 5 was placed between the grating 

 and the plate at a distance of 177*6 centim. from the latter. 

 This method was employed by Pfluger, and though there 

 are some objections to it, it gave fairly concordant results* 

 A portion of the light from the grating passed through the 

 prism, and a portion through a clear place in the quartz plate. 

 There were thus formed two spectra slightly displaced with 

 reference to one another, and by measuring the distance 

 between corresponding lines, the deviation due to the cyanin 

 prism could be calculated. The lateral displacement was 

 very slight, but by using great care in measuring the shift, 

 fairly accurate values for the refractive indices were obtained. 

 The photographic work was extended into the blue and 

 violet portions of the spectrum, in order that a comparison 

 with the more accurate spectrometer determinations could be 

 made, and close agreement was found. The results are given 



in the following table :- 



rism No. 



10, Angle 2' 51 



X. 



fi. 



423 



1-530 



410 



1-565 



404 



1-573 



395 



1-606 



380 



1-600 



372 



1-610 



•5. 



These values, though not shown on the curve, will be found 

 to fit very well. 



Beyond wave-length 372 nothing appeared to be trans- 

 mitted through the cyanin, even with a five-hour exposure, 

 though a strong image of the spectrum transmitted by r the 

 uncovered quartz was obtained. Evidently cyanin has a 

 second absorption -band at this point, for the same thing was 

 found in subsequent work with the interferometer, the absorp- 

 tion of glass not beginning until somewhat further along in 

 the spectrum (about *00033). Whether this absorption 



