196 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
bine in the violet rays) in a repetition of the primary tints in their order, beyond the 
Newtonian spectrum, and that if by any concentration rays still further advanced in 
the “chemical” spectrum could be made to affect the eye with a sense of light and 
colour, that colour would be green, blue, &c., according to the augmented refran- 
gibility. 
190. Cases of negative Photographic Action on Vegetable Tints. — Among a collec- 
tion of plants which I made at the Cape of Good Hope, and have succeeded in rear- 
ing in England, occurred three species of a genus allied to Anthericum, with brilliant 
yellow flowers in lengthened spikes, and highly characteristic furred anthers, to which 
I am not botanist enough to assert the correct application of the name Bulbine, as- 
signed to them by a friend in Cape Town. Of these three species, two ( Bulbine 
bisulcata and ) yield from the green epidermis of their leaves and 
flower-stalks a bright yellow juice, which darkens rapidly on exposure to light, 
changing at the same time to a ruddy brown. Exposed to the spectrum, the less re- 
frangible rays are found inoperative, either in inducing the change of tint, or in pre- 
serving that portion of the paper on which they fall from the influence of dispersed 
light. The negative action commences at the fiducial yellow, is very feeble as far as 
+ 10, where it begins to increase, and is strong at + 23, where the maximum of 
effect is situated. Hence it degrades more slowly, is still pretty strong at + 60, and 
may be traced as far as 80, being therefore nearly commensurate with the spectrum 
impressed on nitro-argentine paper, a range of action unique, so far as my experience 
goes in vegetable photography. The species experimented on is that which (sup- 
posing it undescribed) I should be disposed to call triangularis, from the angular 
section of its long, slender, smooth, solid leaves ; which, with the singular character 
of its juice, may serve to identify the species, my own specimen (a single one) having 
been destroyed by insects after flowering superbly. The ultimate tint acquired by 
the juice is a deep brown, to which it also passes in darkness, but much more slowly. 
The juices of both species, however, have the same photographic characters. 
191. Cheiranthus cheiri, IVall-Jlower. — A cultivated double variety of this flower, 
remarkable for the purity of its bright yellow tint, and the abundance and duration 
of its flowers, yields a juice when expressed with alcohol, from which subsides, on 
standing, a bright yellow, uniform, finely divided fecula, leaving a greenish yellow 
transparent liquid, only slightly coloured, supernatant. The fecula spreads well on 
paper, and is very sensitive to the action of light, but appears at the same time to 
undergo a sort of chromatic analysis, and to comport itself as if composed of two 
very distinct colouring principles, very differently affected. The one on which the 
intensity and sub-orange tint of the colour depends is speedily destroyed, but the 
paper is not thereby fully whitened. A paler yellow remains as a residual tint, 
and this, on continued exposure to light, so far from diminishing in tone, slowly 
darkens to brown. Exposed to the spectrum, the paper is first speedily reduced 
nearly to whiteness in the region of the blue and violet rays. More slowly, an insu- 
