1^8 



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

 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 Vvhich exhibit the 

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

 sion are highly sensitive, may be mentioned a v.-eak 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 madder 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 different 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 is 

 yellow, or yellowish orange. 



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

 very high degree, the mode in which light is dispersed internally 

 presents some very remarkable peculiarities. One of the most sin- 

 gular examples occurs in the case of an alcoholic solution of the 

 green colouring matter of leaves. This fluid disperses a rich red 

 light. The dispersion commences abruptly about the fixed line B, 

 and continues from thence onwards throughout the visible spectrum 

 and a little beyond. The dispersion is subject to fluctuations inti- 

 mately connected with the singular absorption bands exhibited by 

 this medium. 



In order that a medium should be capable of changing the refran- 

 gibility of light incident upon it, it is not necessary that the me- 

 dium should be a fluid, or a clear solid. Washed papers and other 

 opake substances produce the same efl'ect, but of course the mode of 

 observation must be changed. The author has observed the change 

 of refrangibility in various ways, it will be sufficient to mention 

 here that which was found most generally useful, which he calls 

 the method of observing by a linear spectt^wn. The method is as 

 follows. 



A series of prisms and a lens are arranged in the usual manner 

 for forming a pure spectrum, but the slit by which the light enters, 

 instead of being parallel, is placed in a direction perpendicular to 

 the edges of the prisms. A linear spectrum is thus formed at the 

 focus of the lens, consisting of an infinite succession of images of 

 the slit arranged one after the other in the order of refrangibility, 

 and of course overlapping each other to a certain extent. The sub- 

 stance to be examined is placed in the linear spectrum, and the line 

 of light seen upon it is viewed through a prism held to the eye. In 

 this way it is found that almost all common organic substances, such 

 as wood, cork, paper, calico, bone, ivory, horn, wool, quills, feathers, 

 leather, the skin of the hand, the nails, are sensitive in a greater or 

 less. degree. Organic substances w^hich are dark-coloured are fre■^ 



