THE ANOMALOUS DISPERSION OF CYANIN. 



PAET I. 



HISTORICAL SURVEY. 



(a) Experimental. — It has long been a well known fact that 

 prisms having the same angle of refraction, but made of differ- 

 ent kinds of glass, yield spectra which are wholly unlike one 

 another in character. Not only is the refraction or the devia- 

 tion of the light produced by one prism different from that 

 which results when a prism of different material is employed, 

 but the amount of dispersion, or the angular extent of the spec- 

 trum, depends likewise upon the material of which the refracting 

 substance is composed. Moreover, to a very great extent, re- 

 fraction and dispersion are independent of each other, so that 

 media are frequently found having a high refractive index but 

 a small dispersive power, and conversely, other media exist 

 having small refractive power but possessing high dispersion. 

 The angular distance between any two given wave lengths in 

 the prismatic spectrum depends, therefore, not only upon the 

 refracting angle of the prism but upon the nature of the ma- 

 terial of which the prism is made, so that in general, prismatic 

 spectra are unlike one another both in angular extent and in 

 angular distance between corresponding wave lengths. This 

 variability in the character of prismatic spectra is known as the 

 irrationality of dispersion. 



Until the year 1840 no exception had been found to the gen- 

 eral law that short waves are deviated more than longer ones, 

 that is to say, that the order of arrangement of colors in the 

 spectrum is always the same although the distances between any 



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