LIGHT, therefore, is MOTION ; or shall we say that light is the means by which a blow is transmitted from the luminous body to the body upon which light is incident.
Light travels in vacuo with uniform velocity ; but there are different kinds of light, that is, light which exhibits different colours, viz. red, orange, yellow, green, blue, indigo, violet. These different colours are produced by the different lengths of the waves of light, as exhibited in the following table : Hence it appears that the waves of red light being the -longest, the number of undulations in a given time are the fewest ; and the waves of violet light being the shortest, its undulations are the quickest.
When light passes from vacuum into a transparent medium, or from a rare medium into a denser, the velocity of the waves is diminished, and vice-vered. The index of refraction, "ii," in geometrical optics, expresses in physical optics the ratio which the velocity of a wave of light in vacuo bears to its velocity in the medium into which it passes. This quantity " la" is greater for violet than red light ; it would appear, therefore, that their velocities being equal at incidence, the red ray travels faster through a refract ing medium than the violet ray. There would consequently appear to be a connection existing between the length of a wave and the velocity of its propagation. This circumstance is stated as a difficulty at page 285 of Professor Airy's Tract on the Undulatory' Theory of Light. The difficulty has, however, been since removed by Professor Powell, of Oxford, who has demonstrated that within a refracting medium there is actually a difference between the velocities of red and violet light, the condition being that the intervals between the vibrating molecules of ether should bear a sensible ratio to the length of an undulation, which condition is fulfilled within the refracting medium, although apparently not in space, where the velocity of light of all colours is the same.
All material bodies are supposed to be more or less elastic, their particles not being in actual contact, and the interstices between them filled with lumeniferous ether. It is easy to conceive therefore
that the chemical phenomena of light, and we may add of heat, and probably electricity, are produced by motion among the particles of the ether within the interstices of bodies, which communicates motion to the material atoms of the body itself, and alters their mutual arrangement. On this supposition, there c,an be no such thing as latent heat, latent light, or latent electricity, any more than there can be latent motion, which is a contradiction in terms. If we suppose light, heat, actinism, and the various forms of elec tricity, when developed in any body to be nothing more than the motion of an ether pervading all space, and filling the interstices of every substance, but varying in the length, velocity, and species of its undulations, we may explain by one general hypothesis a vast variety of astonishing phenomena due to agents between which many strong analog,ies are found to exist. He,at, for instance, is proved to be the undulation of an elastic medium, and its rays can be reflected, refracted, polarized, and made to exhibit interference just in the same way as rays of light. In short, there is a high deg,ree of probability that the actinic, calorific, and luminous properties of the sunbeam are due simply to the different lengths of the undu lations which are transmitted—a /ong wave (comparatively speaking) like the red exhibiting in a marked degree the effects due to heat, a short wave like the violet those due to actinism, and a wave of medium length, those due to light.
The laws of the reflexion and refraction of light can be easily explained on the undulatory theory, but not without having recourse to a mathematical demonstration which is not sufficiently elementary for the present work. The reader is referred for this demonstration to Professor Airy's Tract, pages 277 to 296, and also to Herschel's " Tteatise on Light." We shall now consider some of the phenomena of POLARIZED