114  SirJ.F. W. Herschel on the Action of the Rays 
colour its tincture imparts to paper, instead of red is a fine 
blue, very slightly verging on slate-blue. But it has by no 
means the ordinary chemical characters of blue vegetable co- 
lours. Carbonate of soda, for instance, does not in the least 
degree turn the expressed juice green; and when washed 
with the mixture, a paper results of a light slate-gray, hardly 
at all inclining to green, ‘The blue tincture is considerably 
sensitive, and from the richness of its tone and the absence 
of residual tint, paper stained with it affords photographic 
impressions of great beauty and sharpness, some of which will 
be found among the collection submitted with this paper for 
inspection. 
199. Senecio splendens.—This flower yields a rich purple 
juice in great abundance and of surprising intensity. Nothing 
can exceed the rich and velvety tint of 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 complete 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 interesting 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 deve- 
lopment, 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 colour, in consequence of 
which I shall content myself with describing the phanomena 
as exhibited on that of the elder leaf. The type of the im- 
pressed spectrum in this case is, as in fig. 8, exhibiting a 
strong decided maximum of action, giving rise to a nearly in- 
sulated solar image at — 11:5, 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, + 68), with a slight in- 
termediate 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 declines gradually, 
and beyond + 45 ceases to be traceable. Photographic pic- 
tures may be taken readily on such papers, half an hour in 
