LIGHT. What is light P Why does it render objects visible, exhibit different colours, produce chemical changes in bodies, and so forth ? These are questions of the highest importance in science, and of especial interest to the photographer. We shall endeavour briefly to explain in the present article what light is, and why it produces many curious phenomena of which no photographer should be ignorant.
Two theories have been advanced with respect to the physical nature of light, one called the " Corpuscular," the other the " Undulatory" theory. On the former hypothesis it was supposed that a luminous body emits particles of light, just as a fowling piece discharges a volley of small shot, and that these minute particles, after travelling through space with immense velocity, at length impinge upon bodies. On the undulatory theory it is sup posed that a luminous body communicates undulations to an elastic ether which pervades space, that these undulations are transmitted with prodigious velocity, and that they constitute light.
We speak of the corpuscular theory in the past tense because it has now been abandoned by men of science as incapable of explaining some of the most important phenomena of light ; and we speak of the undulatory theory in the present tense, because it is now generally adopted, and considered to rest on quite as satisfactory evidence as the Law of Universal Gravitation. In the preface to Professor Airy's " Tract on the Undulatory Theory of Light," he expresses himself thus strongly on the subject:— "The undulatory theory of optics is presented to the reader as having the same claims to his attention as the Theory of Gravita tion,—namely, that it is certainly true, and that by mathematical operations of general elegance it leads to results of great interest. With regard to the evidence for this theory ; if the simplicity of a hypothesis which explains with accuracy a vast variety of phenomena of the most complicated kind, can be considered a proof of its T correctness, I believe there is no physical theory so firmly established as the theory in question. This can be felt completely, perhaps, only by the person who has both observed the phenomena and made the calculations ; as to my own pretensions to the former qualifica tion, I shall merely state that I have repeated nearly every experi ment alluded to in the following tract. This character of certainty, I conceive to belong only to what may be called the geometrical part of the theory ; the hypothesis, namely, that light consists of undula tions depending on transversal vibrations, and that these travel with certain velocities in different media, according to the laws here explained. The mechanical part of the theory, as the supposition.s
relative to the constitution of the ether, the computation of the inteney of reflected and refracted rays, &c., though generally pro bable, I conceive to be far from certain." Such are the opinions of the Astronomer Royal, expressed in the year 1831, with respect to the certainty of the undulatory theory of light ; but since that time some points of difficulty in the hypothesis have been cleared up, and it now rests on a firmer basis than ever. In fact the Corpuscular Theory is now completely exploded, and we allude to it as a matter of history, as we might to any exploded system of Astronomy. Should the reader, therefore, find in any popular treatise on Optics the Corpuscular Theory treated with any sort of gravity or respect, or in any way than as a delusion, he may infallibly conclude that the author is but imperfectly acquainted with his subject.
Light, then, is the undulation of an ether which pervades space, just as Sound is the undulation of the air ; but both the media and the species of undulation are different in the two cases. Air is a material ponderable substance,—lumeniferous ether, an imponderable substance, and therefore not strictly material according to the defini tion of matter (see " Imponderable Agents") ; still it may be, and no doubt is material, although it has not actually been proved to possess weight. Should space be filled with a material ether, how ever subtle, it would act as a resisting medium to the motions of the heavenly bodies, and in the case of the comets which have but little mass, and move with enormous velocity, its effect as a direct force continually opposing their motion would be to cause them to describe smaller orbits in an appreciably shorter period of time. Now it is found that the periods of the comets of Encke and Biela are actually diminished by a few hours in every revolution round the Sun ; but this may happen in consequence of their passing through nebulous matter which appears to surround the Sun for a considerable distance, so that it does not afford absolute proof of the materiality of the lumeniferous ether, although it increases the probability of its being subject to the common laws of grosser matter.