484 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
Tincture of Turmeric. 
42. This fluid is very sensitive, and exhibits a pretty copious dispersive reflexion 
of a greenish light. In its mode of internal dispersion it strongly resembles a solution 
of guaiacum, but the final tint of the dispersed light does not correspond to so high a 
mean refrangibility. When the fluid was examined by the third method, the true 
dispersion appeared to commence about b. The absorbing power was so great for 
the rays of high refrangibility, that from a little above F (in the case of tincture not 
diluted with alcohol) to the end the dispersed light seemed to be confined to the mere 
surface. By the fourth method the dispersion was as usual traced a little lower down 
in the spectrum. When the dispersed beam was first perceived it was nearly homo- 
geneous, and its refrangibility was only a very little less than that of the active light. 
As the refrangibility of the active light increased, new colours, in the order of their 
refrangibility, entered into the dispersed beam, which became more and more com- 
posite, while at the same time its upper limit became distinctly separated from the 
beam of falsely dispersed light, which, when the whole dispersed beam was analysed 
by a prism, was always found in advance of the other. The tint of the dispersed 
beam passed from orange through yellow to yellowish green, which was its final tint. 
Tincture of turmeric is well adapted for exhibiting the fixed lines in the invisible part 
of the spectrum, though perhaps not quite so well as a solution of sulphate of quinine. 
Alcoholic Extract from the Seeds of the Datura Stramonium. 
43. This fluid, which I was led to try in consequence of Sir David Brewster’s 
paper, proved to be remarkably sensitive, and exhibited a copious dispersive reflexion 
of a pale but lively green. The general phenomena are so nearly the same as in a 
solution of sulphate of quinine that there is no need of a separate description. The 
principal difference consists in the tint, which is green instead of blue. In the pre- 
sent case, however, the fluid, in addition to its dispersion of green, dispersed a red 
beam under the influence of certain red rays. As the lens employed in the fourth 
method of examination was moved from the extreme red onwards, the light was at 
first inactive, but when the lens reached a certain point of the spectrum, a red beam 
of truly dispersed light suddenly appeared, which disappeared with almost equal sud- 
denness as the lens moved on. In this mode of observation the refrangibility of the 
dispersed could hardly be distinguished from that of the active light ; but on com- 
bining the first and third methods, by removing the lens, placing the vessel truly in 
focus, and holding a blue glass alternately in front of the vessel and in front of the 
eye, I satisfied myself that the truly dispersed beam, taken as a whole, was of lower 
refrangibility than the light by which it was produced. The utility of the blue glass 
depended upon the circumstance that the upper extremity of the extreme red which 
it transmitted nearly coincided with the point of the spectrum at which the red beam 
occurred. This red beam was doubtless due to the presence of a small quantity of 
