252 



BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



yet been found: "Coming from the red end of the spectrum 

 and approaching an absorption band the refractive index is ab- 

 normally increased, and coming from the violet end towards the 

 absorption band the refractive index is abnormally decreased." 

 To make the peculiar variations in the refractive index di- 

 rectly visible to the eye, Kundt employed the method of crossed 

 prisms 1 as first used by Xewton 2 in ordinary dispersion. A 

 beam of white light is first dispersed by a glass prism placed 

 with its refracting edge vertical. This spectrum is then passed 

 through a hollow prism filled with an alcoholic solution of 

 cyanin and placed with its refracting edge horizontal. Thus 

 the prisms have their refracting edges at right angles to each 

 other so that the first one deviates the light in a horizontal plane 

 while the second bends the rays in a vertical direction. Sup- 

 pose the deviation is simply inversely proportional to the wave 

 length, that is, a normal spectrum, such as is produced by a 

 grating, in each case, the resulting crossed prism spectrum will 

 become a diagonal band having the violet end elevated. This 

 is seen in (a), Fig. 2. The lower band shows the appear- 

 ance of the spectrum produced by the first prism only, while 



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the diagonal band represents the final result after passing 

 through both prisms. If now the dispersion of the second 

 prism was irrational, as is the case with ordinary glass, the final 

 result would be a curved band as is seen in the upper part of 

 (b), Fig. 2. If the common glass prism is removed and a 

 hollow prism filled with a cyanin solution, as used by Kundt, 



I Kundt, Pogg. Ann. CXLIV., p. 133. 

 a Newton's Opticks, Book I, Prop. II. 



