OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
199 
same reagent, the tint is fuller and richer, as well as, photographically, more sensi- 
tive, and the residual yellow less abundant. And in this case the resistance of the 
tint to rays of its own colour is very strongly marked. The spectral impression con- 
sists, in fact, of two portions clearly separated by the whole of the interval occupied 
by the green and greenish blue rays, conformably to the general remark in Art. 
170. 
197- Sparaxis tricolor ?, var. — Stimulating effect of alkalies . — Among a great many 
hybrid varieties of this genus, lately forwarded to me from the Cape, occurred one of 
a very intense purplish brown colour, nearly black. The alcoholic extract of this 
flower in its liquid state is rich crimson brown. Spread on paper it imparted a dark 
olive green colour, which proved perfectly insensible to very prolonged action, either 
of sunshine or the spectrum. The addition of carbonate of soda changed the colour 
of this tincture to a good green, slightly inclining to olive, and which imparted the 
same tint to paper. In this state, to my surprise, it manifested rather a high degree 
of photographic sensibility, and gave very pretty pictures with a day or two of expo- 
sure to sunshine. When prepared with the fresh juice there is hardly any residual 
tint, but if the paper be kept, a great amount of indestructible yellow remains out- 
standing. The action is confined chiefly to the negative end of the spectrum, the 
maximum being at — 8'0, and the sensible limits of the impression (corrected for 
semidiameter) being — 1 TO and + 56*4, of which, however, all but the first five or 
six parts beyond the fiducial yellow show little more than a trace of action. A pho- 
tograph impressed on this paper is reddened by muriatic acid fumes. If then trans- 
ferred to an atmosphere of ammonia, and when supersaturated the excess of alkali 
allowed to exhale, it is fixed, and of a dark green colour. Both the tint and sharp- 
ness of the picture, however, suffer in this process. 
198. Red Poppy— Papaver Rheum}. — Among the vegetable colours totally de- 
stroyed by light, or which leave no residual tint, at least when fresh prepared, 
perhaps the two most rich and beautiful are those of the red poppy, and the double 
purple groundsel ( Senecio splendens ). The former owes its red colour in all proba- 
bility to free carbonic acid, or some other (as the acetic) completely expelled by dry- 
ing, for the 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 cha- 
racters of blue vegetable colours. 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 resi- 
dual 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 abun- 
dance and of surprising intensity. Nothing can exceed the rich and velvety tint of 
