Royal Society. 391 



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 Uttle 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- 

 gibihty 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 eflPect, 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 spectrum. 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 which are dark-coloured are fre- 

 quently found to be insensible, but, on the other hand, scarlet cloth 

 and various other dyed articles are highly sensitive. By means of a 

 linear spectrum the peculiar dispersion of a red light produced by 

 chlorophyll, or some of its modifications, may be observed not only 

 in a solution, but in a green leaf, or on a washed paper, or in a sea- 

 weed. 



The highly sensitive papers obtained by washing paper \^^th tinc- 

 ture of turmeric, or a solution of sulphate of quinine, or some other 

 highly sensitive medium, display their sensibihty in a remarkable 

 manner when they are examined in a linear spectrum. In these 

 cases, however, the paper produces a very striking effect when 

 merely held so as to receive a pure spectrum formed in the usual 

 manner, that is, with a slit parallel to the edges of the prisms. Such 

 a paper may be used as a screen for showing the fixed hues belonging 

 to the invisible rays, though they are not thus shown quite so well 

 as by using a solution. The extraordinary prolongation of the spec- 

 trum seen when it is received on turmeric paper, has been already 

 observed by Sir John Herschel, by wliom it was attributed to a pecu- 

 liarity in the reflecting power of that substance. Of course it now 

 appears that the true explanation is very diflTerent. 



