PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 487 
dried by pressing them between sheets of blotting paper. Nettle was chosen partly 
because it stands boiling without losing its green colour, and partly for other reasons. 
My object in boiling the leaves was to obtain the green colouring matter more nearly 
in a state of isolation, but it seems to have the additional advantage of giving a 
solution less liable to decomposition. Indeed, this fluid seemed disposed to remain 
permanently unchanged when kept in the dark ; but a small portion of it which was 
exposed to strong light had its colour rapidly discharged. 
48. When fresh leaves are left in contact with alcohol in the dark, or in only weak 
light, the colour of the fluid changes by degrees, and it seems to approximate 
(making allowance for impurities) to a type which is nearly represented by the fluid 
obtained in this manner from laurel leaves, or that obtained by treating with alcohol 
tea leaves from which a good deal of brown colouring matter has first been extracted 
by water. This type was rather ideal than actual, being derived from a comparison 
of different cases, until it seemed to be realized in the case of a fluid obtained by re- 
dissolving in alcohol a crust which had formed itself at the bottom of a test tube 
containing leaf-green. The principle to which the peculiar absorption and internal 
dispersion of such a fluid seems due may be called modified leaf-green. The fluid 
itself is not green but olive-coloured, becoming red at great thicknesses. 
49. When solutions of leaf-green, and of its various modifications, are examined in 
different thicknesses by the light of a candle, there are five bands of absorption which 
may be observed in the spectrum. These will be called, in the order of their refran- 
gibility. Nos. 1, 2, 3, 4 and 5, the bright bands below the respective dark bands 
being also numbered in the same manner. Of the dark bands. Nos. 1, 2, 3 and 5, 
are the first four in Sir David Brewster’s plate*. No. 4 is mentioned in the memoir, 
but not represented in the plate, which corresponds to a thickness not sufficient to 
bring out this band. The last band in the plate could not be seen without strong 
light. The dark bands Nos. 1 and 2 are situated in the red. No. 3 about the yellow 
or greenish yellow. No. 4 in the green, and No 5 early in the blue. Of these. No. 1 
is in small thicknesses by far the most intense, and it may be readily seen even in a 
very dilute solution ; it might apparently be used as a chemical test of chlorophyll, or 
one of its modifications. The test would be of very easy application, since it would be 
sufficient to hold a test tube with the liquid at arm’s length before a candle at a little 
distance, and view the linear image of the flame through a prism applied to the eye. 
50. Fresh and modified leaf-green differ much in the order in which the bright 
bands are absorbed, and in the degree to which the dark bands are developed before 
they cease to be visible by the absorption of the part of the spectrum in which they 
are situated. In the green fluid, the dark band No. 5 is not usually seen, because 
the spectrum is there cut off, unless a very small thickness be used. With a mode- 
rate thickness. Nos. 2 and 3, especially the former, are well seen, and No. 1 is very 
intense. As the absorption goes on, the bright bands Nos. 2 and 3 are absorbed, 
* Edinburgh Transactions, vol. xii. 
3 R 
MDCCCLII. 
