200 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
paper tinted with it while fresh. It is, however, exceedingly insensible to light, and 
it is only by an exposure continued for many weeks, that it is possible to get a com- 
plete photographic impression of a picture on it. Still, when obtained, owing to the 
whiteness of the ground, the effect is pleasing, and would be beautiful were it not 
that the general tint suffers somewhat in its tone and softness of surface. 
200. The juices of the leaves, stalks, roots, & c. of plants afford a wide and inter- 
esting field of photographic inquiry. Those of leaves are for the most part green, 
and being usually loaded with gum, extractive, &c., are difficult of manipulation. 
Such as I have tried, which spread well on paper, as the elder, the potatoe, the night- 
shade, and a few others, proved very sensitive if gathered when just in the perfection 
of their development, and in full vitality. As the season advances they lose much of 
their sensibility. There is much uniformity in the action of the spectrum on their co- 
lour, in consequence of which I shall content myself with describing the phenomena 
as exhibited on that of the elder leaf. The type of the impressed spectrum in this 
case is, as in fig. 8, exhibiting a strong decided maximum of action, giving rise to a 
nearly insulated solar image at — 1T5, or almost at the extremity of the red rays. 
The colour of this image was a pale yellowish pink or flesh colour ; from thence the 
action is feeble, with two subordinate minima (at — 5 - 0, -f- 6'8), with a slight inter- 
mediate maximum at 0‘0, and beyond these (or about the termination of the green) 
the action again increases ; reaches another maximum at + 20‘0, after which it de- 
clines gradually, and beyond -f- 45 ceases to be traceable. Photographic pictures 
may be taken readily on such papers, half an hour in good sun sufficing ; but the glairy 
nature of the juices prevents their being evenly tinted, and spoils their beauty*. 
201. The ruddy tint which comes out when the green is destroyed by light, is in 
all probability that which gives the whole colour to sere and withered leaves, whe- 
ther simply disclosed by the destruction of the green which masked it in the live 
state of the leaf, or matured by exposure to light during the whole season, either out 
of the elements of the green colouring matter destroyed, or from the other juices of 
the vegetable. It deserves to be noticed in connexion with this, that all the lively 
vegetable greens have a large portion of red in their composition, and are in fact di- 
chromatic. A good example of such a colour is a solution of sap-green, which, used 
as a prism, is seen to transmit both red and green rays, separating them by a broad 
interval which increases as the thickness or density of the solution is increased; the 
red ultimately preponderating, and the green being extinguished. If we view a 
garden or shrubbery through a glass of a pure and deep red colour, every shrub, such 
as the laurel, of a lively and brilliant foliage, and especially green grass, will appear 
scarlet. Under such circumstances, a grass-plot, seen in contrast with a gravelled 
walk, shows as light on darkness, contrary to their habitual order of illumination. 
So gi;eat is the quantity of extreme red light reflected by a green sward, as actually 
* I have not operated on chlorophyle (the green colouring matter of leaves) in a state of purity, owing to 
the nicety required in its preparation. 
