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were able to penetrate before they were absorbed. This distance 

 was at first considerable, greater than the diameter of the vessel, but 

 it decreased with great rapidity as the refrangibility of the incident 

 rays increased, so that from a little beyond the extreme violet to the 

 end the blue space was reduced to an excessively thin stratum adja- 

 cent to the surface by which the incident rays entered. It appears 

 therefore that this fluid, which is so transparent with respect to 

 nearly the whole of the visible rays, is of an inky blackness with 

 respect to the invisible rays more refrangible than the extreme violet, 

 The fixed lines belonging to the violet and the invisible region beyond 

 were beautifully represented by dark planes interrupting the blue 

 space. When the eye was properly placed, these planes were of 

 course projected into lines. The author has made a sketch of these 

 fixed lines, which accompanies the paper. They may be readily 

 identified with the fixed lines represented in M. Becquerel's map of 

 the fixed lines of the chemical spectrum. The last line seen in a 

 solution of sulphate of quinine appears to be the line next beyond 

 the last represented in M. Becquerel's map. Under very favourable 

 circumstances two dusky bands were seen still further on. Several 

 circumstances led the author to conclude that in all probabilit)'' fixed 

 lines might be readily seen corresponding to still more refrangible 

 rays, were it not for the opacity of glass with respect to those rays 

 of very high refrangibility. 



It is very easy to prove experimentally that the blue dispersed 

 light corresponding to any particular part of the incident spectrum 

 is not homogeneous light, having a refrangibility equal to that of the 

 incident rays, and rendered visible in consequence of its complete 

 isolation ; but that it is in fact heterogeneous light, consisting of 

 rays extending over a wide range of refrangibility, and not passing 

 beyond the limits of refrangibility of the spectrum visible under 

 ordinary circumstances. To show this it is sufficient to isolate a 

 part of the incident spectrum, and view the narrow beam of dispersed 

 light which it produces through a prism held to the eye. 



In Sir David Brewster's mode of observation, the beam of light, 

 which was of the same nature as the blue light exhibited by a solu- 

 tion of sulphate of quinine, was necessarily mixed with the beam due 

 merely to reflexion from suspended particles ; and in the case of 

 vegetable solutions, a beam of the latter kind almost always exists, to 

 a greater or less degree. But in the method of observation employed 

 by the author, to which he was led by the discovery of the change 

 of refrangibility, the two beams are exhibited quite distinct from one 

 another. The author proposes to call the two kinds of internal 

 dispersion just mentioned true internal dispersion and false internal 

 dispersion, the latter being nothing more than the scattering of light 

 which is produced by suspended particles, and having, as is now 

 perfectly plain, nothing to do with the remarkable phenomenon of 

 true internal dispersion. 



Now that the nature of the latter phenomenon is better known, it 

 is of course possible to employ methods of observation by which it 

 may be detected even when only feebly exhibited. It proves to be 

 Proceedings of the Royal Society, Vol, VI. No. 89. 14 



