516 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
consisting mainly of a red band. Some red wool, dyed I suppose with madder, proved 
extremely sensitive. The derived spectrum in this case was pretty broad, but red was 
the predominant colour. Green wool, dyed I do not know wdth what, was also very 
sensitive, giving a pretty broad derived spectrum, in which green was the predomi- 
nant colour. These examples may suffice, but the reader must not suppose that they 
form the only instances in which dispersion was observed among dyed substances. 
On the contrary, it is extremely common in this class. 
131. Brazilwood, safflower, red sandal wood, fustic and madder, all gave rise to 
solutions having a pretty high degree of sensibility. The solutions here referred to 
were such as were obtained directly by water, &c., in which the colours which these 
substances are capable of producing were not brought out. The beautiful red colour- 
ing matters of logwood and camwood appear to be insensible ; for a fresh-made solu- 
tion of logwood in water exhibited no perceptible sensibility, and the slight sensibility 
exhibited by a similar solution of camwood seemed to have no relation to the red 
colouring matter. 
132. Paper washed with a solution of madder in alcohol was sensitive in a pretty 
high degree, but the sensibility was greatly inereased by afterwards washing with a 
solution of alum. Accordingly I found that a decoction of madder in a solution of 
alum exhibited a very high degree of sensibility, displaying a copious dispersive 
reflexion of a yellow light. In this medium the dispersion commenced about the 
fixed line D, and continued from thence onwards far beyond the extreme violet, so 
that the group of fixed lines n was seen with great ease. 
133. Safflower red, examined in the shape in which it is sold on what is called a 
pink saucer, proved highly sensitive, giving a bright and narrow derived spectrum, 
which consisted chiefly of the more refrangible red. This substance possesses some 
other remarkable optical properties, which however do not belong to the immediate 
subject of this paper. 
134. Metals proved totally insensible. I have examined gold, platinum, silver, 
mercury, copper, iron, lead, zinc and tin. Brass is like simple metals in this respect ; 
but if the surface be lackered the lacker displays its own sensibility. 
135. The non-rnetallic elements, carbon, sulphur, iodine and bromine, are in- 
sensible. 
136. Among common stones I have found dark flint, limestone, chalk and some 
others which were sensitive, though only in a low degree compared with organic 
substances. To guard against any impurity of the surface, the stones were broken 
across, and the fresh surface examined. In the cases mentioned, the sensibility 
observed is not to be attributed to the chief ingredient of the stone, for quartz, chal- 
cedony, Iceland spar and Carrara marble were insensible. 
