MAGNUSSON — ANOMALOUS DISPERSION OF CYANIN. 287 



The two photographs used for tables XI and XII were taken 

 when cyanin film \'<>. 1 1 1 was in one path of the interferometer. 

 The relation between /a and a, as given by tables XI and 

 XII, is shown graphically by the line in plate IS, while the 

 broken line represents the results obtained by direct spectrometer 

 readings. The fringes in figure 1 of plate 22 are not quite 

 parallel, and hence the errors in measuring a and b are 

 larger, therefore the results for this film are not as good as for 

 film Xo. II. 



Conclusion. — It was originally intended to attempt a more 

 rigorous proof of the Ketteler-IIelmholtz dispersion formula, 

 by using more accurately determined values of the refractive 

 indices of cyanin for waves of various lengths. Pfliigers work 

 while confirming the formula for the visible part of the spec- 

 trum, showed discrepancies in the ultra violet and extreme red. 

 He suggests that these discrepancies, at least in the ultra violet, 

 may be caused by the presence of an ultra violet obsorption band. 

 This band has unquestionably been found, but until its charac- 

 teristics have been more thoroughly determined (a matter re- 

 quiring a large amount of further experimental work) any at- 

 tempt to apply the new values to the formula would be useless. 



The most important results which have been obtained by this 

 work may be summarized as follows: 



1st. A fairly accurate determination of the dispersion curve 

 for cyanin from the extreme red well into the ultra violet. 



2d. The conclusive evidence furnished by the photographs 

 taken with the interferometer, of the continuity of the curve 

 through the absorption band of cyanin. 



3d. The discovery of a new absorption band in the ultra vio- 

 let spectrum of cyanin, the character of which will have to be 

 accurately determined before any conclusive proof of the Ket- 

 teler-Helmholtz dispersion theory can be furnished by measure- 

 ments upon this substance. 



This work was undertaken two years ago, at the suggestion 

 of Professor R. W. Wood, and is a continuation of a series of 



