390 Royal Society. 



circumstances two dusky bands were seen still further on. Several 

 circumstances led the author to conclude that in all probability 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 coiTespondiug 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 sulficient 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 b}"^ 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 

 almost universal in vegetable solutions, that is, in solutions made 

 directly from various parts of vegetables. When vegetable products 

 are obtained in a state of isolation, their solutions sometimes ex- 

 hibit the phenomenon and sometimes do not, or at least exhibit it 

 so feebly that it is impossible to say whether what they do show 

 may not be due to some impurity. Among fluids which exhibit the 

 phenomenon in a high degree, or according to the author's expres- 

 sion are highly sensitive, may be mentioned a weak decoction of the 

 bark of the horse-chestnut, an alcoholic extract from the seeds of 

 the Datura stramonium, weak tincture of turmeric, and a decoction 

 of maddet in a solution of alum. In these cases the general cha- 

 racter of the dispersion resembles that exhibited by a solution of 

 sulphate of quinine, but the tint of the dispersed light, and the part 

 of the spectrum at which the dispersion begins, are diiferent in dif- 

 ferent cases. In the last fluid, for example, the dispersion com- 

 mences somewhere about the fixed line D, and continues from thence 

 onwards far beyond the extreme violet. The dispersed light ia 

 yellow, or yellowish orange. 



In the case of other fluids, however, some of them sensitive in a 



