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most refrangible rays, but reaches much beyond the extreme red. On paper prepared with formobenzoate of silver the chemical spectrum is cut off at the orange rays, with phosphate of silver in the yellow, and with chloride of gold it terminates with the green, with carbonate of mercury it ends in the blue, and on paper prepared with the percyanide of gold, ammonia, and nitrate of silver, the darkening lies entirely beyond the visible spectrum at its most refrangible extremity, and is only half its length, whereas in some cases chemical action occupies a space more than twice the length of the luminous image.
The point of maximum energy of chemical action varies as much for different preparations as the scale of action. In the greater number of cases the point of deepest blackening lies about the lower edge of the indigo rays, though in no two cases is it exactly the same, and in many substances it is widely different. On paper prepared with the juice of the ten-week stock (Mathiola annua) there are two maxima, one in the mean yellow and a weaker in the violet; and on a preparation of tartrate of silver Sir John Herschel found three, one in the least refrangible blue, one in the indigo, and a third beyond the visible violet. The decrease in photographic energy is seldom perfectly alike on both sides of the maximum. Thus at the most refrangible end of the solar spectrum the greatest chemical power is exerted in most instances where there is least light and heat, and even in the space where both sensibly cease.
Not only the intensity but the kind of action is different in the different points of the solar spectrum, as evidently appears from the various colours that are frequently impressed on the same analyzing surface, each ray having a tendency to impart its own colour. Sir John Herschel obtained a coloured image of the solar spectrum on paper prepared according to Mr. Talbot's principle, from a sunbeam refracted by a glass prism and then highly condensed by a lens. The photographic image was rapidly formed and very intense, and, when withdrawn from the spectrum and viewed in common daylight, it was found to be coloured with sombre but unequivocal tints imitating the prismatic colours, which varied gradually from red through green and blue to a purplish black. After washing the surface in water, the tints became more decided by being kept a few days in the dark—a phenomenon, Sir John observes, of constant
occurrence, whatever be the preparation of the paper, provided colours are produced at all. He also obtained a coloured image on nitrate of silver, the part under the blue rays becoming a blue brown, while that under the violet had a pinkish shade, and sometimes green appeared at the point corresponding to the least refrangible blue. Mr. Hunt found on a paper prepared with fluoride of silver that a yellow line was impressed on the space occupied by the yellow rays, a green band on the space under the green rays, an intense blue throughout the space on which the blue and indigo rays fell, and under the violet rays a ruddy brown appeared; these colours remained clear and distinct after being kept two months.
Notwithstanding the great variety in the scale of action of the solar spectrum, the darkening or deoxydizing principle that prevails in the more refrangible part rarely surpasses or even attains the mean yellow ray which is the point of maximum illumination; it is generally cut off abruptly at that point which seems to form a limit between the opposing powers which prevail at the two ends of the spectrum. The bleaching or oxydizing effect of the red rays on blackened muriate of silver discovered by M. Ritter of Jena, and the restoration by the same rays of discoloured gum guaiacum to its original tint by Dr. Wollaston, have already been mentioned as giving the first indications of that difference in the mode of action of the chemical rays at the two ends of the visible spectrum, now placed beyond a doubt.
The action exerted by the less refrangible rays beyond and at the red extremity of the solar spectrum, in most instances, so far from blackening metallic salts, protects them from the action of the diffused daylight: but, if the prepared surface has already been blackened by exposure to the sun, they possess the remarkable property of bleaching it in some cases, and under other circumstances of changing the black surface into a fiery red.
Sir John Herschel, to whom we owe most of our knowledge of the properties of the chemical spectrum, prepared a sheet of paper by washing it with muriate of ammonia, and then with two coats of nitrate of silver; on this surface he obtained an impression of the solar spectrum exhibiting a range of colours very nearly corresponding with its natural hues. But a very remarkable phenomenon occurred at the end of least refrangibility; the red rays exerted a protecting influence which pre
served the paper from the change which it would otherwise have undergone from the deoxydizing influence of the dispersed light which always surrounds the solar spectrum, and this maintained its whiteness. Sir John met with another instance on paper prepared with bromide of silver, on which the whole of the space occupied by the visible spectrum was darkened down to the very extremity of the red rays, but an oxydizing action commenced beyond the extreme red, which maintained the whiteness of the paper to a considerable distance beyond the last traceable limit of the visible rays, thus evincing decidedly the existence of some chemical power over a considerable space beyond the least refrangible end of the spectrum. Mr. Hunt also found that on the Daguerreotype plate a powerful protecting influence is exercised by the extreme red rays. In these cases the red and those dark rays beyond them exert an action of an opposite nature to that of the violet and lavender rays.
The least refrangible part of the solar spectrum possesses also, under certain circumstances, a bleaching property, by which the metallic salts are restored to their original whiteness after being blackened by exposure to common daylight, or to the most refrangible rays of the solar spectrum.
Paper prepared with iodide of silver, when washed over with ferrocyanite of potash, blackens rapidly when exposed to the solar spectrum. It begins in the violet rays and extends over all the space occupied by the dark chemical rays, and over the whole visible spectrum down to the extreme red rays. This image is coloured, the red rays giving a reddish tint and the blue a blueish. In a short time a bleaching process begins under the red rays, and extends upwards to the green, but the space occupied by the extreme red is maintained perfectly dark, Mr. Hunt found that a similar bleaching power is exerted by the red rays on paper prepared with protocyanide of potassium and gold with a wash of nitrate of silver.
The application of a moderately strong hydriodate of potash to darkened photographic paper renders it peculiarly susceptible of being whitened by further exposure to light. If paper prepared with bromide of silver be washed with ferrocyanate of potash while under the influence of the solar spectrum, it is immediately darkened throughout the part exposed to the visible rays down to the end of the red, some slight interference being
perceptible about the region of the orange and yellow. After this a bleaching action begins over the part occupied by the red rays, which extends to the green. By longer exposure an oval spot begins again to darken about the centre of the bleached space; but, if the paper receive another wash of the hydriodate of potash, the bleaching action extends up from the green, over the region occupied by the most refrangible rays and considerably beyond them, thus inducing a negative action in the most refrangible part of the spectrum.
In certain circumstances the red rays, instead of restoring darkened photographic paper to its original whiteness, produce a deep red colour. When Sir John Herschel received the spectrum on paper somewhat discoloured by exposure to direct sunshine, instead of whiteness, a red border was formed extending from the space occupied by the orange, and nearly covering that on which the red fell. When, instead of exposing the paper in the first instance to direct sunshine, it was blackened by the violet rays of a prismatic spectrum, or by a sunbeam that had undergone the absorptive action of a solution of ammoniasulphate of copper, the red rays of the condensed spectrum produced on it, not whiteness, but a full and fiery red, which occupied the whole space on which any of the visible red rays had fallen; and this red remained unchanged, however long the paper remained exposed to the least refrangible rays.
Sunlight transmitted through red glass produces the same effect as the red rays of the spectrum in the foregoing experiment. Sir John Herschel placed an engraving over a paper blackened by exposure to sunshine, covering the whole with a dark red-brown glass previously ascertained to absorb every ray beyond the orange: in this way a photographic copy was obtained in which the shades were black, as in the original engraving; but the lights, instead of being white, were of the red colour of venous blood, and no other colour could be obtained by exposure to light, however long. Sir John ascertained that every part of the spectrum impressed by the more refrangible rays is equally reddened, or nearly so, by the subsequent action of the less refrangible; thus the red rays have the very remarkable property of assimilating to their own colour the blackness already impressed on photographic paper.
That there is a deoxydating property in the more refrangible
rays, and an oxydating action in the less refrangible part of the spectrum, is manifest from the blackening of one and the bleaching effect of the other; but the peculiar action of the red rays in the experiments mentioned shows that some other principle exists different from contrariety of action. These opposite qualities are balanced or neutralized in the region of the mean yellow ray. But, although this is the general character of the photographic spectrum, under certain circumstances even the red rays have a deoxydating power, while the blue and violet exert a contrary influence; but these are rare exceptions.
The photographic action of the two portions of the solar spectrum being so different, Sir John Herschel tried the effect of their united action by superposing the less refrangible part of the spectrum over the more refrangible portion by means of two prisms; and he thus discovered that two rays of different refrangibility, and therefore of different lengths of undulation, acting simultaneously, produce an effect which neither, acting separately, can do.
Some circumstances that occurred during the analysis of the chemical spectrum seem to indicate an absorptive action in the sun's atmosphere. The spectral image impressed on paper prepared with nitrate of silver and Rochelle salt commenced at, or very little below, the mean yellow ray, of a delicate lead colour; and when the action was arrested, such was the character of the whole photographic spectrum. But, when the light of the solar spectrum was allowed to continue its action, there was observed to come on suddenly a new and much more intense impression of darkness, confined in length to the blue and violet rays; and, what is most remarkable, confined also in breadth to the middle of the sun's image, so far at least as to leave a border of the lead-coloured spectrum traceable, not only round the clear and well-defined convexity of the dark interior spectrum at the less refrangible end, but also laterally along both its edges; and this border was the more easily traced, and less liable to be mistaken, from its striking contrast of colour with the interior spectrum, the former being lead gray, the latter an extremely rich deep velvety brown. The less refrangible end of this interior brown spectrum presented a sharply terminated and regularly elliptical contour, the more refrangible a less decided one. "It may
seem too hazardous," Sir John continues, "to look for the cause