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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
of this very singular phenomenon in a real difference between the chemical agencies of those rays which issue from the central portion of the sun's disc, and those which, emanating from its borders, have undergone the absorptive action of a much greater depth of its atmosphere; and yet I confess myself somewhat at a loss what other cause to assign for it. It must suffice, however, to have thrown out the hint, remarking only, that I have other, and I am disposed to think decisive, evidence of the existence of an absorptive solar atmosphere extending beyond the luminous one." M. Arago observed that the rays from the centre of the sun have a greater photographic power than those from the edges, and the photographic images of the sun, taken on glass by M. Niepcé, were blood-red, much deeper in the centre, and on one occasion the image was surrounded by an auriol. Several circumstances concur in showing that there are influences also concerned in the transmission of the photographic action which have not yet been explained, as, for example, the influence which the time of the day exercises on the rapidity with which photographic impressions are made, the sun being much less effective two hours after passing the meridian than two hours before. There is also reason to suspect that the effect in some way depends on the latitude, since a much longer time is required to obtain an image under the bright skies of the tropics than in England; and it is even probable that there is a difference in the sun's light in high and low latitudes, because an image of the solar spectrum, obtained on a Daguerreotype plate in Virginia, by Dr. Draper, differed from a spectral image obtained by Mr. Hunt on a similar plate in England. The inactive spaces discovered in the photographic spectrum by M. E. Becquerel, similar to those in the luminous spectrum, and coinciding with them, is also a phenomenon of which no explanation has yet been given; possibly the chemical rays may be absorbed by the atmosphere with those of light. Although chemical action extends over the whole luminous spectrum, and much beyond it, in gradations of more or less intensity, it is found by careful investigation to be by no means continuous; numerous inactive lines cross it, coinciding with those in the luminous image as far as it extends; besides, a very great number exist in the portions that are obscure, and which overlap the visible part. There are three extraspectral lines beyond the red, and some strongly marked groups on the
obscure part beyond the violet; but the whole number of those inactive lines, especially in the dark spaces, is so great that it is impossible to count them.
Notwithstanding this coincidence in the inactive lines of the two spectra, photographic energy is independent of both light and heat, since it exerts the most powerful influence in those rays where they are least, and also in spaces where neither sensibly exist; but the transmission of the sun's light through coloured media makes that independence quite evident. Heat and light pass abundantly through yellow glass, or a solution of chromate of potash; but the greater part of the chemical rays are excluded, and chlorine gas diluted with common air, though highly pervious to the luminous and calorific principles, has the same effect. Sir John Herschel found that a slight degree of yellow London fog had a similar effect with that of pale yellow media: he also remarked that a weak solution of azolitmine in potash, which admits a great quantity of green light, excludes chemical action; and some years ago the author, while making experiments on the transmission of chemical rays, observed that green glass, coloured by oxyde of copper about the 20th of an inch thick, excludes the photographic rays; and, as M. Melloni has shown that substance to be impervious to the most refrangible calorific rays, it has the property of excluding the whole of the most refrangible part of the solar spectrum, visible and invisible. Green mica, if not too thin, has also the same effect, whereas amethyst, deep blue, and violet-coloured glasses, though they transmit a very little light, allow the chemical rays to pass freely. Thus light and photographic energy may be regarded as distinct parts of the solar beam, and both being propagated by vibrations of the etherialmedium they are merely motion. Excellent images have been obtained of the moon in its different phases by Professor Secchi, at Rome; candlelight is nearly deficient of the chemical rays. How far they may influence crystallization and other molecular arrangements is unknown, but their power is universal wherever the solar beam falls, although their effect only becomes evident in cases of unstable molecular equilibrium.
It is not by vision alone that a knowledge of the sun's rays is acquired: touch proves that they have the power of raising the temperature of substances exposed to their action. Sir William Herschel discovered that rays which produce the sensation of heat
exist in the solar spectrum independent of those of light; when he used a prism of flint glass, he found that the warm rays are most abundant in the dark space a little beyond the red extremity of the spectrum, that from thence they decrease towards the violet, beyond which they are insensible. It may be concluded therefore, that the calorific rays vary in refrangibility, and that those beyond the extreme red are less refrangible than any rays of light. Since Sir William Herschel's time it has been discovered that the calorific spectrum exceeds the luminous one in length in the ratio of 42 to 25, but the most singular phenomenon is its want of continuity. Sir John Herschel blackened the under side of a sheet of very thin white paper by the smoke of a lamp, and, having exposed the white side to the solar spectrum, he drew a brush dipped in spirit of wine over it, by which the paper assumed a black hue when sufficiently saturated. The heat in the spectrum evaporated the spirit first on those parts of the paper where it fell with greatest intensity, thereby restoring their white colour, and he thus discovered that the heat increases uniformly and gradually throughout the luminous spectrum, and that it comes to a maximum and forms a spot at a considerable distance beyond the extreme red. It then decreases, but again increasing it forms a second maximum spot, after which it ceases altogether through a short space, but is again renewed and forms two more insulated spots, and even a fifth may be traced at a little distance from the latter. These circumstances are probably owing to the absorbing action of the atmospheres of the sun and earth. "The effect of the former," says Sir John, "is beyond our control, unless we could carry our experiments to such a point of delicacy as to operate separately on rays emanating from the centre and borders of the sun's disc; that of the earth's, though it cannot be eliminated any more than in the case of the sun's, may yet be varied to a considerable extent by experiments made at great elevations, under a vertical sun, and compared with others where the sun is more oblique, the situation lower, and the atmospheric pressure of a temporarily high amount. Should it be found that this cause is in reality concerned in the production of the spots, we should see reason to believe that a large portion of solar heat never reaches the earth's surface, and that what is incident on the summits of lofty mountains differs not only in quantity but also in quality from what the plains receive.
A remarkable phosphorescent property was discovered by M. E. Becquerel in the solar spectrum. Two luminous bands separated by a dark one are excited by the solar spectrum on paper covered with a solution of gum arabic, and strewed with powdered sulphuret of calcium or Canton's phosphorus. One of the luminous bands occupies the space under the least refrangible violet rays, and the other that beyond the lavender rays, so that the dark band lies under the extreme violet and lavender rays. When the action of the light is continued, the whole surface beyond the least refrangible violet shines, the luminous bands already mentioned brightest; but all the space from the least refrangible violet to the extreme red remains dark. If the surface, prepared either with the sulphuret of calcium or Bologna stone, be exposed to the sun's light for a little time, it becomes luminous all over; but when, in this state, a solar spectrum is thrown upon it, the whole remains luminous except the part from the least refrangible violet to the extreme red, on which space the light is extinguished; and when the temperature of the surface is raised by a lamp, the bright parts become more luminous and the dark parts remain dark. Glass stained by the protoxide of copper, which transmits only the red and orange rays, has the same effect with the less refrangible part of the spectrum; hence there can be no doubt that the most refrangible and obscure rays of the spectrum excite phosphorescence, while all the less refrangible rays of light and heat extinguish it.
Paper prepared with the sulphuret of barium, when under the solar spectrum, shows only one space of maximum luminous intensity, and the destroying rays are the same as in the sulphuret of calcium. Thus the obscure rays beyond the extreme violet produce light, while the luminous rays extinguish it.
The phosphoric spectrum has inactive lines which coincide with those in the luminous and chemical spectra, at least as far as it extends; but in order to be seen the spectrum must be received for a few seconds upon the prepared surface through an aperature in a dark room, then the aperture must be closed, and the temperature of the surface raised two or three hundred degrees; the phosphorescent parts then shine brilliantly and the dark lines appear black. Since the parts of similar refrangibility in different spectra are traversed by the same dark lines, rays of the same refrangibility are probably absorbed at the same time by the different media through which they pass.