PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILTJT OF LIGHT. 515 
green did belong to the red colouring matter ; but till I had obtained this matter in 
solution I did not feel certain that it might not have been due to chlorophyll, the 
spectrum of which exhibits a division in the green. 
126. When this fluid was examined in Sir David Brewster’s manner, and the 
dispersed beam was analysed, the spectrum was found to consist of a broad band 
like that which has been already described as seen in the derived spectrum given by 
a frond of red sea-weed. When the solution, which happened to be very weak, was 
examined by the third method, the dispersion was found to be produced chiefly by a 
portion of the incident spectrum, having a breadth about equal to that of the interval 
between the two principal bands of absorption. To each of these bands corresponded 
a maximum of activity. The tint of the dispersed light was nearly uniform ; but by 
the fourth method of observation some faint dispersed red could be made out, which 
appeared before the main part of the dispersion had come on. This medium affords 
a very good example of an intimate connexion between absorption and internal 
dispersion. 
127. The colouring matters of birds’ feathers appeared to be insensible, white 
feathers being most sensitive, pale ones next, and dark ones not at all : however, I 
have not examined a large collection. 
128. Of coloured fruits, such as currants, &c., the colouring matter appeared, in 
the very few cases which I have examined, to be quite insensible. 
129. A set of water colours were by no means remarkable for sensibility, but rather 
the contrary. The inorganic colours appeared quite insensible, except white lead, 
the sensibility of which was perhaps due to size, and offered nothing striking, either 
as to its character or as to its amount. Some lakes and other organic colours proved 
moderately sensitive. But I found one water colour, called Indian yellow, which 
stands pretty high among sensitive substances. In its mode of dispersion it much 
resembles turmeric, but it does not come up to that substance in the amount of 
sensibility. It is said to be composed of urate of lime, but I do not know how far it 
may be regarded as chemically pure. 
130. Many of the substances used in dyeing, and dyed articles in common use, 
furnish very remarkable examples of sensibility. Archil, litmus and turmeric have 
been already mentioned ; and I have been recently informed by a friend that the 
Mercurialis perennis, in which a striking instance of sensibility was observed, was 
formerly employed in dyeing. A piece of scarlet cloth, examined in a linear spec- 
trum, gave a copious derived spectrum which was very narrow, consisting chiefly of 
the more refrangible red. With a vertical slit the bands H and fixed lines beyond 
were seen on a red ground. Paper washed with a solution of cochineal and after- 
wards with a solution of alum, when examined in a linear spectrum, displayed a pretty 
high degree of sensibility, the derived spectrum consisting in this case of a red band. 
If tartaric acid be used instead of alum, the dispersion is a good deal more copious. 
Common red tape is another example in which the derived spectrum is very copious. 
