If one of the dark heated bodies of which we have been speaking, be placed before a concave metallic mirror, we find that there is a focus of heat produced by reflection; and this focus is formed at precisely the same point with that of luminous rays proceeding from the same body. The calorific emanation, therefore, is reflected specularly like light, making the angle of reflection equal to the angle of incidence; and as the specular reflection of light is nothing or nearly nothing, from rough bodies, when it does not fall very obliquely upon their surface; so there are certain surfaces which reflect obscure caloric with different degrees of intensity. It is strongly reflected, for example, from the surface of polished metals, but much less from the surface of glass, however perfectly polished. It was for this reason that M. Berard made use of a metallic mirror in the experiment above mentioned.

We have considered only the colorific and calorific properties of light. We shall now speak of its chemical effects. This point also has been accurately investigated by M. Berard. Chemists had long observed that when the muriate of silver and other white salts are exposed to the influence of light, they soon become black. Gum guaiacum, exposed thus to light, passes from yellow to green, as has been observed by Dr Wollaston. MM. Gay-Lussac and Thénard have made known an action of this kind still more immediate and energetic; for, upon exposing to a solar beam a mixture of hydrogen gas and chlorine in equal volumes, a detonation immediately took place, the product of which was hydrochloric acid, heretofore called muriatic acid. M. Berard made use of these different substances as reagents, in examining and ascertaining the chemical properties of the different rays of the spectrum. For having placed in the spaces occupied by the different colours, smal! pieces of paper impregnated with muriate of silver, or small vessels filled with a mixture of the two gases, he was enabled to judge of the energy of the action of the different rays by the intensity and rapidity with which chemical changes took place in the substances thus exposed. He ascertained in this manner, that the chemical action was really most intense towards the violet extremity of the spectrum, and that they extended as had been maintained by M. Ritter and Dr Wollaston, a little beyond this extremity. Moreover, when these substances were exposed for a certain time to the action of each ray, which could be easily done on account of the immobility of the spectrum, he succeeded in observing sensible effects, though continually decreasing in intensity, in the indigo and blue rays; whence he considered it probable that if reagents still more sensible were employed, analogous effects

might be observed, though more feeble, in the other rays. In order to render evident the great disproportion which exists, in this respect, between the energies of the different rays, M. Berard concentrated, by means of a lens, all that part of the spectrum which is comprised between the green and extreme violet, and he concentrated in a similar manner, by another lens, all that portion which extends from the green to the other extremity. This last portion was collected into one point sensibly white, and so dazzling that the eye could hardly endure it. Nevertheless the muriate of silver remained exposed for more than two hours to this vivid light, without experiencing any sensible alteration. On the contrary, when exposed to the other portion which was much less white and dazzling, in less than ten minutes the muriate was found to become black. M. Berard concluded from this experiment that the chemical effects produced by light are not owing solely to the heat which it developes in bodies, by combining with their substance; since, on this supposition, the power of producing chemical combinations might be expected to be most intense in the rays, which possessed, in the highest degree, the power of producing heat. But perhaps we shall find less opposition between these two considerations, if we bear in mind, that, according to the experiments of De Laroche, there may be essential differences between the obscure caloric employed by chemists, to alter certain combinations, particularly the vegetable colours, and the caloric of the spectrum in the part which does not produce these effects. For example, the difficulty would not exist, if the obscure caloric, obtained by artificial heat, were wholly or partly analogous to the equally obscure emanations which take place at the violet extremity of the spectrum, and there does not seem to be any impossibility in such a supposition.

These experiments of Mrard leave no doubt that the different portions of a solar ray, dispersed by the prism, have very different properties as to the power which they possess of producing vision, heat, and chemical combination. Are we then to attribute these three powers to three distinct kinds of rays, existing independently of each other, and capable each of producing only one single effect. If this be the case, it will also be necessary that each of these species should be capable of being separated by the prism into an infinity of different modifications, like light itself, since we find by experiment that each of the three properties, chemical, illuminating, and calorific, is distributed, though in very unequal proportions, over a certain extent of the spectrum. Thus, according to this hypothesis, we must conceive three spectrums, one calorific, one chemical, and one luminous, superposed upon each other. It must likewise be admitted that Opt. 42

each of the substances which compose the spectrums, and even each of the particles of unequal refrangibility which compose these substances, is endued, like the particles of visible light, with the property of being polarised by reflection, and of escaping the influence of the reflecting force, as is observed with respect to the luminous particles, and in the same cases. A like analogy must exist also in the other properties. But instead of this complicated system, we suppose, conformably to the phenomena, that the solar light is composed of an assemblage of rays unequally refrangible, and consequently capable of being differently modified by the action of bodies, which supposition requires that there should be original differences, either in their masses, their velocities, or their affinities. Why should these rays which differ in so many respects, all produce upon thermometers and upon our organs, the same effects as to heat and light? Why should they have the same power in forming and separating chemical combinations? Is it not very natural to suppose that vision may take place in our eyes only between certain limits of refrangibility, and that too great or too small a refrangibility may render the rays equally unfit to produce this effect. It is possible that these rays are capable of exciting a vision in other eyes; perhaps they are capable of exciting it in certain animals; on this supposition all that is marvellous in the above phenomena disappears, or rather it becomes a part of the general action of light. In a word, we may suppose the calorific and chemical properties to vary throughout the whole extent of the spectrum with the refrangibility, but according to different functions, in such a manner that the calorific property shall be at its minimum at the violet extremity of the spectrum, and at its maximum at the red extremity; while, on the contrary the chemical faculty, expressed by another function, shall have its minimum at the red extremity, and its maximum at the violet extremity or a little beyond it. This single supposition, which is the simplest representation of the phenomena, perfectly satisfies all the facts stated above, and even enables us to predict a great number of them from analogy alone. Indeed, if all the rays which produce vision, heat, and chemical combinations, are equally rays of light, they must all necessarily be reflected from polished bodies, and reflected according to the same law, making the angle of reflection equal to the angle of incidence; whence it follows that they will be, in like manner, concentrated or dispersed by concave or convex mirrors. They must, moreover, be all polarised in traversing a crystal endued with the property of double refraction, or in being reflected from glass, ice, &c., under a determinate incidence; and when they have received these modifications, they must

be reflected from another surface of the same kind, if it be placed in such a manner as to render its reflecting force efficacious upon the luminous particles. On the contrary, if this force produces no effect upon the visible luminous particles, invisible light will no longer be reflected; for the cause which determines or prevents reflection, appears to act equally upon all the particles, whatever be their refrangibility; and must therefore operate upon invisible light, since the condition of visibility or invisibility has reference only to the constitution of our eyes, and not to the nature of the particles themselves which produce the sensation in us. Finally, since, according to the observations of De Laroche, obscure caloric, emanating from bodies gradually heated, approximates gradually to the conditions and properties belonging to luminous caloric, we might suppose that when the emanation begins to become visible, it would be immediately analogous to the least calorific part of the spectrum, which is the extremity of the violet. Accordingly we observe that flame of whatever kind, in its incipient state, is violet or blue, and only attains to whiteness when it has reached a high degree of intensity. Still this view of the subject from the very circumstance that it supposes a progressive state, is not inconsistent with the idea of particular properties, belonging exclusively to a particular stage of the progression. Thus the calorific emanations of different temperatures, and the luminous emanations of different colours, may differ from each other in the property of producing vision, heat, and chemical action, in the power of being transmitted through transparent substances, and perhaps in many other particulars which we have not yet dis

covered.

IV.

Measure of the Intensities of Light.

It often happens in optical researches, that we have occasion to compare the intensities of two lights presented at the same time or successively. In the first case, which is the most simple, we illuminate separately with these two lights, equal discs of very white paper, or some other unpolished body which is a good reflector; then, viewing at the same time the two discs, we remove the most intense of the two lights, until they appear equally bright. Hence the intensities will be as the squares of the distances of the lights from the discs. This partial illumination may be obtained by illuminating a white space with these two lights, and interposing before them a

small opaque disc, the shades of which will indicate the points separately illuminated. It is sufficient, then, to render these shades equally deep, by varying the distance of the luminous body from the screen. We may also admit the two lights separately through Fig. 180. two conical tubes united at their vertices, and terminated at this place by two equal discs of white paper. Then viewing both discs at once, having the head covered for the purpose of excluding all foreign light, we give notice to an assistant which of the two luminous objects must be removed or brought nearer, until the two discs appear equally bright. When we have arrived at this result, the intensities of the two lights are proportional to the squares of the distances from their respective discs.

But if we wish to compare two lights which are not visible at the same time, we have only to select a third, whose brightness is of such a nature as to sustain itself without variation, and compare it successively with each of the other two. By employing these processes and various others of a similar kind, Bouguer obtained a variety of curious results from which the following are selected.

Table of the quantities of light reflected by the surface of water under different obliquities.

Table of the quantities of light reflected by the first surface of polished