130 EPIPOLIC PHENOMENA. 



coloured with the oxide of uranium, and by a decoc- 

 tion of the inner bark of the horse-chesnut tree. Mr. 

 Stokes, who has investigated this class of phenomena, 

 andproposestocallitF/worescerace, from its being naturally 

 seen in fluor-spar, has shown that the peculiar internal 

 dispersion, and the consequent alteration of the colour 

 of the ray, is due to an alteration in its refrangibility. 

 Whether this hypothesis prove to be the correct one or 

 not, it is certain that there exists a set of rays of far 

 higher refrangibility than those seen in the ordinary 

 Newtonian spectrum. This may be shown in the follow- 

 ing manner : taking either of the solutions named, or a 

 block of uranium glass, throw upon one face, by means of 

 a prism, a very pure spectrum. On looking into the 

 glass or fluid there will be seen, commencing amidst the 

 most refrangible rays, a new set of spectral rays, strug- 

 gling to make their way through the absorbent medium. 

 These are of a blue colour in the quinine or chesnut 

 solution, and green in the uranium glass, and are seen 

 extending themselves far beyond the most refrangible 

 rays of the ordinary Newtonian spectrum. This is the 

 space over which those rays which have the power of pro- 

 ducing chemical changes, such as are rendered familiar 

 by the practice of Photography, are detected in their 

 greatest activity. It has, therefore, been supposed that 

 these fluorescent rays are the chemical rays rendered lu- 

 minous by the alteration of their refrangibility. This 

 view has received much support from the fact that the 

 extra spectral rays are crossed with numerous dark 

 lines, and that in the chemical impressions these 

 lines are marked by unchanged spaces which exactly 

 coincide with them. There is, however, much doubt of 

 the correctness of this, since, in the uranium glass of 

 such a thickness that these visible rays are quite ab- 

 sorbed, the chemical rays still pass. 



However, the whole question requires, and is receiving, 

 the most searching investigation. The discovery of 



