low colour, and in consequence, the spectrum thrown on it, if exposed in the open daylight, is considerably affected in its apparent colours, the blue portion appearing violet, and the violet very pale and faint; but beyond the region occupied by the violet rays is distinctly to be seen a faint prolongation of the spectrum, terminated latterally, like the rest of it, by straight and sharp outlines, and which in this case affects the eye with the sensation of a pale yellow colour. Comparative measures were carefully taken of the spectrum so prolonged, and of the ordinary spectrum as seen projected on white paper, the results being as follows (see Plate I. fig 6.): Length of the spectrum Y L. from the fiducial point Y to the visible termination L, as seen (with the naked eye) on the turmeric paper; corrected for moon's semidiameter Length Y V from the same fiducial point to the visible termination, as similarly seen when projected on white paper .... Prolongation rendered visible by projection of the spectrum on turmeric paper....... PART. 56.6 =40·4 } =16.2 Com 187. The day on which this experiment was orst made (May 27, 1841) was serene and clear, but being aware that in certain states of the atmosphere a vertical beam of halo-light passes through the sun, which in a meridional position of that luminary might give rise to perceptible prolongation both upwards and downwards (though in fact no such prolongation was perceived at the red end), it was often repeated, and always with the same result, on subsequent occasions, whether the sun were on or near the meridian, or otherwise. parative trials, also with other yellow papers, fully satisfied me of the cause being traceable to a peculiarity in the colouring material, as to its reflective powers. In particular, a certain paper (No. 1055.) coloured with the juice of Chryseis californica, whose tint was almost identical with that of the turmeric paper, only somewhat brighter, was tried, and the spectrum measured on this paper was found to terminate precisely at 440, i. e. (correcting for semidiameter) at 40.4, the very same as if white paper had been used. 188. To test the matter yet more pointedly, a strip of turmeric paper was fixed on the Chryseis paper, so that its edge should bisect the spectrum longitudinally from end to end, the preceding half of the sun's lengthened image being received on the one paper, and the following half on the other. The papers thus arranged were so similar as hardly to be distinguished when simply laid in sunshine, but when illuminated by the spectrum, as above described, the half of it on the turmeric side was plainly seen to extend far beyond the other, as represented in fig 6. 189. Hitherto I have met with only one other coloured paper which possesses a similar character in respect of its reflective power, and that by no means in so high a degree. To prepare it, the alcoholic tincture of the dark purple dahlia must be alkalized by carbonate of soda. The mixture is vivid green, which is also, at first the colour of paper stained with it. But this colour changes in about twenty-four hours to a fine yellow, a little inclining to orange, after which it is remarkably permanent, and very little sensible to photographic impression. On this, as on the turmeric paper, the prolongation of the spectrum appears as a pale yellow streak. And if such, rather than lavender or dove-colour, should be the true colorific character of these rays, we might almost be led to believe (from the evident reappearance of redness mingled with blue in the violet rays) in a repetion of the primary tints in their order, beyond the Newtonian spectrum, and that if by any concentration rays still furter 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 refrangibility. 190. Cases of negative Photographic Action on Vegetable Tints.Among a collection of plants which I made at the Cape of Good Hope, and have succeeded in rearing 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, assigned 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 refrangible rays are found inoperative, either in inducing the change of tint, or in preserving that portion of the paper on which they fall from the influence of dispersed light. The negative action commences at the fiducial yellow, it 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 (supposing 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, Wall-flower.-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 fæcula, leaving a greenish yellow transparent liquid, only slightly coloured, supernatant. The fæcula 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 insulated solar image is whitened at - 10-5, or in the less refrangible portion of the red, and the impressed spectrum assumes the type represented in fig. 7. where m Y = — 10·5; m' Y = + 13·0; Yc+55. The exposure continuing, a brown impression begins to be perceived in the midst of the white streak, which darkens very slowly from 18-6 to 42, It never attains any great intensity, but presents a singular appearance in the midst of a white train previously eaten out. 192. The juice in question contains gallic acid, and probably tannin, as is evident from its striking a strong black with persalts of iron. The gallic acid itself (whose singular properties, in conjunction with nitrate of silver, have been developed by Mr. Talbot, as the basis of his all but magical process of the calotype)* is affected also negatively by light. Paper washed with its spirituous solution, and partially covered, being exposed several months in a window, was found pretty strongly darkened in all the exposed portion. The action is too slow for prismatic analysis, and I am far from attributing to the presence of this acid the phenomenon above recorded. It would rather appear as if some portion of a more decidedly negative ingredient analogous to that which exists in the Bulbine, were present. As regards the positive ingredient, I may mention here the common Marigold (in which also the colour resides in an insoluble fæcula) as a flower in which the colouring principle is probably identical both with this and with that of the Corchorus Japonica, since it comports itself in the very same manner under the spectrum, -is nearly, or quite as sensitive, and is moreover fugitive, even when carefully defended from light, giving photographs which cannot be preserved. Many other flowers also contain in their juices a portion of this identical, or a very similar yellow principle, probably in a state of greater solubility, and thence disposed to the absorption of oxygen. Thus the juice of a fine purely yellow species of Mimulus,† if Preparations of the gallic acid in conjunction with silver, are noticed by me in my former paper as forming a "problematic exception" to my general want of success in procuring at the very outset of my photographic experiments (in February 1839), papers more sensitive than the simple nitrated or carbonated ones. The problematic feature consisted in spontaneous darkening of the papers laid by to dry in the dark, so at least then considered, but really arising doubtless from light incident on them in their preparation. Acetate of silver was used in their preparation. + Mimulus Smithii (Lindl.) expressed, with or without alcohol, though vividly yellow in the first moments of expression, passes almost instantly to dirty green, and loses its sensibility to light; but if crushed on paper and iminediately dried, the petals give a bright yellow stain which agrees in sensibility, and in the type of the impressed spectrum with the Corchorus. The Ferranca undulata, a dark brown flower, yields, when expressed, a dull green juice, which, spread on paper and dried, turns very speedily blue under the influence of the blue and violet rays of the spectrum; owing to the destruction of this yellow principle, which, mingled with the substratum of blue (itself a much more indestructive tint), gives it its natural tinge of green. A similar destruction, of probably again the same yellow matter, in the colour of the American Marygold,* causes its tint to pass rapidly in sunshine from brown to green, after which continued exposure produces no further change. The yellow colour of fresh bees' wax and of palm oil, are also, I doubt not, referable to the same, or a nearly similar colouring matter, both being very speedily bleached by exposure to light. 193. Viola adorato.-Chemists are familiar with the colour of this flower as a test of acids and alkalies, for which, however, it seems by no means better adapted than many others; less so, indeed, than that of the Viola tricolor, the common purple Iris, and many others might be named. It offers, in fact, another, and rather a striking instance of the simultaneous existence of two colouring ingredients in the same flower, comporting themselves differently, not only in regard to light but to chemical agents. Extracted with alcohol, the juice of the violet is of a rich blue colour, which it imparts in high perfection to paper. Exposed to sunshine, a portion of this colour gives way pretty readily, but a residual blue, rather inclined to greenish, resists obstinately, and requires a very much longer exposure (for whole weeks indeed) for its destruction, which is not even then complete. Photographic impressions, therefore, taken on this paper, though very pretty, are exceedingly tedious in their preparation, if we would have the lights sharply made out. 194. The residual tint thus outstanding, after long exposure, is turned, not green, but yellow, by alkalies; or, if greenish at first, a very few hours suffice for the destruction of the slight remnant of blue, and the consequent appearance of the yellow colour. Reasoning on this fact, as well as on the action of light above mentioned, it seems highly probable that the tincture in question holds in solution two distinct colouring principles, of which the one (greatly preponderant in quantity) is destructible by light, and either destroyed or turned green by alkalies; the other indestructible by light, and either naturally yellow in colour or changeable into yellow by alkaline agency. 195. This view of the composite nature of the colour in question receives corroboration from the habitudes of the alcoholic tincture * French Marygold, Tagetes Patula. above mentioned, when rendered green by admixture of carbonate of soda. On making this addition it becomes evident that a large amount of colour has been destroyed; the green tint imparted by it to paper being far less intense than might be expected from the intensity of the original hue, and from the trifling dilution caused by the small quantity of alkaline liquid required to effect the change. What remains is a fine green; but when exposed to light, the blue constituent alone of that green is destroyed, and a residual tint of pure yellow, which is very indestructible by light, is left. Exposure of a slip of such paper to the spectrum proves this change to be operated almost wholly by rays less refrangible than the fiducial yellow. A slight discoloration is perceived in the indigoblue rays (at about + 30), but the green appears quite inactive. 196. In the case of the purple Iris mentioned above, when turned green by the same reagent, the tint is fuller and richer, as well as, photographically, more sensitive, 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 consists, 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 tricolour?, var.-Stimulating effect of alkalies.Amongst 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 exposure 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 outstanding. The action is confined chiefly to the negative end of the spectrum, the maximum being at 80, and the sensible limits of the impression (corrected for semidiameter) being 110 and + 564, of which, however, all but the first five or six parts beyond the fiducial yellow show little more than a trace of action. A photograph impressed on this paper is reddened by muriatic acid fumes. If then transferred 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 sharpness of the picture, however, suffer in this process. 198. Red Poppy-Papaver Rheum?.-Among the vegetable colours totally destroyed 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 (Senicio |