PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 513 
116. Paper washed with an ethereal solution from dried archil exhibited very well 
the sensibility of that substance. The derived spectrum consisted chiefly of two 
distinct portions, one containing orange and a little red, the other consisting chiefly 
of green, just as in the beam of dispersed light, produced by white light taken as a 
whole, which the solution itself exhibited. Indeed, I have found that the prismatic 
composition of dispersed light could be determined even more conveniently by means 
of a linear spectrum than by means of the beam dispersed by a solution. 
117. The inside of the capsules of the Datura stramomum is nearly white, and 
apparently uniform. But when the capsules are examined in a linear spectrum, 
certain patches shine out like bright clouds in the invisible rays. The whole of the 
inside is sensitive, as such substances almost always are, but these patches, which 
are probably spots against which the seeds have pressed, are remarkably so. The 
capsules were examined after they had begun to burst. 
118. By means of a linear spectrum the sensibility of chlorophyll may be detected 
in a green leaf. It is exhibited by the appearance in the derived spectrum of a 
narrow pure red band of remarkably low refrangibility. The refrangibility is so low 
that I have always found this band separated from the derived spectrum due to 
other sensitive substances with which chlorophyll or one of its modifications might 
have been mixed. 
119. The petals of flowers, so far as I have examined, are as a class rather 
remarkable for their insensibility, some appearing quite insensible, and others only 
slightly sensitive. The bright yellow chafify involucre of a species of everlasting, 
proved, however, highly sensitive, and its sensibility was also displayed in an alcoholic 
solution. This medium was sensitive enough to exhibit a pretty copious dispersive 
reflexion of a pale greenish yellow light. Its sensibility was more confined than 
usual to the rays of very high refrangibility. 
120. Among petals, the most remarkable which I have observed are those of the 
purple groundsel (Senecio elegans). These petals disperse a red liglit, more copious 
than is usual among petals. If a petal be placed behind a slit, and the transmitted 
light be analysed, it is found to exhibit three remarkable bands of absorption, much 
resembling those of blue glass, but closer together, and beginning later in the 
spectrum, the first appearing about the place of the orange. These bands are still 
better seen in a solution of the colouring matter in weak alcohol. On examining 
this medium by the third method, with a lens of shorter focus than usual, and look- 
ing down from above, the places of the absorption bands were indicated by tooth- 
shaped interruptions in the beam of light reflected from motes. The points of these 
teeth were occupied by red dispersed light, which did not appear in the intervening 
beams of light reflected from motes, from whence it appears that there is the same 
sort of connexion between the absorption and dispersion of this medium as was 
noticed in Art. 59, in the case of solutions of chlorophyll and its modifications. 
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