TEMPERATURE OF PRISMATIC RAYS.

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(596.) The temperature of these rays has been made the next subject of consideration, and it has been shown that the radiations producing HEAT do not coincide with those producing LIGHT. Their points of maximum effects differ widely, and a remarkable extension of Heat rays has been traced over a large space, where there is no probability we shall ever detect Light. The sensations of these two phenomena are dissimilar—the refrangibility of these radiations are different-the physical effects are in every respect unlike each other, therefore it appears scarcely philosophical to group them together as the results of the same cause. It is, important, however, that their points of resemblance should be noticed.

(597.) A remarkable extension of luminous radiations over a space ordinarily regarded as a dark space has been already noticed, and the question with which we have at

mechanics. The emission doctrine, if it means anything, must suppose luminous phenomena to be in analogy with those of ordinary motion; and if the doctrine of undulation means anything, it means that the phenomena of light and sound are alike in their vibratory agitation, and thus the one party likens optics to barology and the other to acoustics. ** No one doubted the mechanical nature of the principal effects of gravity and sound long before the progress of rational dynamics admitted of their exact analysis. The application powerfully tended to the perfection of barology and acoustics; but this was precisely because there was nothing forced or hypothetical about it. It is otherwise with optics. Notwithstanding all arbitrary suppositions, the phenomena of light will always constitute a category sui generis necessarily irreducible to any other: a light will for ever be heterogeneous to a motion or a sound. Again, physiological considerations discredit this confusion of ideas, by the characteristics which distinguish the sense of sight from those of hearing, and of touch and pressure. If we could abolish such distinctions as these by gratuitous hypotheses, there is no saying where we might stop in our wanderings. A chemical philosopher might make a type of the senses of taste and smell, and proceed to explain colours and tones by likening them to flavours and scents. It does not require a wilder imagination to do this, than to issue as a supposition, now become classical, that sounds and colours are radically alike. * Physicists must then abstain from fancifully connecting the phenomena of light and motion."-AUGUSTE COMTE.

present to deal, is the extent or limits of that influence which, when acting upon the organs of vision enables us to distinguish objects. It is well known that some very remarkable cases of idiochromic vision occur. Dr. Dalton and Mr. Troughton are familiar examples; in their eyes, pure red and pure yellow excited the same sensations. Scarlet and green could not be distinguished from each other.* In those instances there does not appear to have been any ray of Light to which the eye was not sensible; but the power of receiving the impression of colour was wanting. This is a diseased condition of the organ, which bears on the point in question. That is; is it possible we may eventually be enabled to show the existence of Light below the extreme red rays, where heat rays are detected ? It will be found, that if several persons, who are looking at the same prismatic spectrum, are requested to mark with the point of a needle the limits of the coloured rays, not many of them will mark the same spot; some see more of the violet or of the red rays than others, and consequently will point a little higher or lower as the case may be. Now, if we protect the eye from the luminous rays by a piece of cobalt-blue glass, we are enabled to see a class of rays that are less refrangible than the ordinary red, and which have been called "the extreme red rays." Sir John Herschel first pointed out the means by which a "lavender grey," ray may be shown to exist beyond the violet. If, indeed, the spectrum is received upon paper, dyed with an alcoholic tincture of turmeric, the spectrum appears elongated beyond the violet ray; but the eye is affected with the sensation of a pale yellow colour. A paper dyed with an alcoholic tincture of the dark purple dahlia, alkalised with carbonate of soda, which becomes in about twenty-four hours a fine yellow colour, exhibits this pro

* Dr. George Wilson, of Edinburgh, is engaged in the investigation of the phenomena of colour-blindness, and we may expect much valuable information from his researches.

longation as a pale yellow streak. "If such," says Sir John Herschel, "should be the true calorific 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 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 refrangibility." Something like this is actually the case, as Mr. Stokes's discoveries have shown.

(598.) It has been supposed that some animals may have the power of distinguishing such colours, that is that their eyes are affected by those rays of high refrangibility, which produce no impression of Light upon the eyes of man. M. Biot has, it would appear, found that some such effect is produced upon the eyes of some of the nightroving animals, by rays invisible to us. This may be. That which is darkness to us when we come from the sunshine into it, is found after a little time, when the lenticular arrangement of the eye has been adjusted to the required condition, to produce the sensation of tolerable Light. May we not, therefore, explain on the same principle, the power which the cat, the owl, and other animals possess of seeing in the circumstances we might regard as darkness? This reasoning is not calculated to settle the question, whether the eyes of any animals receive sensations of Light from the rays of heat, or of those producing chemical change; and it does appear that a broad distinction is established between the solar influence, Heat, and the solar influence, Light. That in many phenomena their operations so run together, that it is impossible to separate the one from the other, I am ready to admit. The experiments of Delaroche have been interpreted as showing that Light and Heat are convertible into one another. The curious fact, discovered by

this philosopher, that radiating heat becomes more and more capable of penetrating glass as the temperature increases, til at a certain temperature the rays become luminous; almost seem to confirm this, did they stand alone. The results obtained by Melloni with the solar rays do, as it appears to me, contrary to his own arguments, compel us to consider Light and Heat as two distinct powers, intimately connected with each other in their operations.

(599.) It is now necessary to examine with great care the question of the identity or otherwise of the chemical principle ACTINISM, which we derive from the same solar source. It must have been remarked, that one of the most striking peculiarities which the prismatic spectrum presents, in its action on sensitive preparations, is the remarkable difference between the scale of action on preparations, not very dissimilar from each other. This is shown in the frontispiece, and reference to it will render it unnecessary to do more than allude to two or three remarkable examples.

(600.) The limits of solar action upon twenty-nine different preparations: mineral and vegetable, is shown in the Plate. The maximum will be found,- marked by the dotted line, -to vary in every instance, but with the exception of three salts of gold (12. 15. and 18.), and the juice of the ten weeks' stock (27.), it will be seen that these maxima are confined within the limits of the least refrangible blue, and the most refrangible violet rays. No. 1. Nitrate of silver shows that the action ceases a little beyond the visible spectrum, and that the rays of the yellow space have but little action; and that those of the red are confined to very small limits. No. 4. Iodide of silver and ferro-prussiate of potash, exhibits the influence of a chemical force to a great distance beyond the luminous spectrum at the violet end. The same influence greatly modified in power is exerted again over the space covered by the red, and extra-spectral calorific rays; and

No. 5., which is a specimen of the action of the hydriodic salts upon the darkened chloride of silver, shows the extension of a purely chemical action considerably lower in the calorific region; and this is even still further extended in No. 22., ferro-cyanate of potash, and per-chloride of iron. In all, but No. 18., the strongest impression is made by the chemical agency of those rays within the limits of the luminous spectrum; but in this remarkable instance, the action is confined entirely to the non-luminous space beyond the violet rays. In No. 14., chloride of gold, this action is entirely confined to the region of the blue, indigo, and violet rays, whilst No. 16., the protocyanide of gold, and formobenzoate of silver; the influence is extended with great force to a considerable distance beyond the violet rays, and a large amount of chemical power is exerted by the rays found in the region of the red space. These, and the other instances, which are described in their proper places, sufficiently show, that the action is extended over the entire luminous spectrum, and spaces occupied by dark rays, nearly equal to twice the length of the visible spectral image. A singular difference is also exhibited in the action of the different spaces on different materials.* In those preparations only which are obtained from the vegetable kingdom do we detect any action over the spaces covered by the yellow rays; the point of maximum illuminating effect.

(601.) By the investigations to which the art of photography has given rise, we have discovered, that the chemical influence is not confined to the class of blue rays, and the dark rays beyond them, as was formerly thought. This chemical power has been traced over every ray of the luminous spectrum, and to some distance beyond its least refracted end, as well as the end of most refrangibility. It is also found that the maximum point, although it may be said to lie within the limits of the most refrangible

* See Appendix.-Becquerel's Experiments.