The fringes are altogether independent of the form or density of the object, being the same when it is round or pointed, when of glass or platinum. When the rays which form the fringes arrive at the screen, they are of different lengths, in consequence of the curved path they follow after passing the edge of the object. The waves are therefore in different phases or states of vibration, and either conspire to form coloured fringes or destroy one another in the obscure intervals. The coloured fringes bordering the shadows of objects were first described by Grimaldi in 1665; but, besides these, he noticed that there are others within the shadows of slender bodies exposed to a small sunbeam, a phenomenon which has already been mentioned to have afforded Dr. Young the means of proving, beyond all controversy, that coloured rings are produced by the interference of light. It may be concluded that material substances derive their colours from two different causes: some from the law of interference, such as iridescent metals, peacocks’ feathers, &c.; others from the unequal absorption of the rays of white light, such as vermilion, ultramarine, blue, or green cloth, flowers, and the greater number of coloured bodies. The latter phenomena have been considered extremely difficult to reconcile with the undulatory theory of light, and much discussion has arisen as to what becomes of the absorbed rays. But that embarrassing question has been ably answered by Sir John Herschel in a most profound paper on the Absorption of Light by coloured Media, and cannot be better given than in his own words. It must, however, be premised, that, as all transparent bodies are traversed by light, they are presumed to be permeable to the ether. He says,—“Now, as regards only the general fact of the obstruction and ultimate extinction of light in its passage through gross media, if we compare the corpuscular and undulatory theories, we shall find that the former appeals to our ignorance, the latter to our knowledge, for its explanation of the absorptive phenomena. In attempting to explain the extinction of light on the corpuscular doctrine, we have to account for the light so extinguished as a material body, which we must not suppose annihilated. It may, however, be transformed; and among the imponderable agents, heat, electricity, &c., it may be that we are to search for the light which has become thus comparatively stagnant. The heating power of the solar rays gives a _primâ facie_ plausibility to the idea of the transformation of light into heat by absorption. But, when we come to examine the matter more nearly, we find it encumbered on all sides with difficulties. How is it, for instance, that the most luminous rays are not the most calorific, but that, on the contrary, the calorific energy accompanies, in its greatest intensity, rays which possess comparatively feeble illuminating powers? These and other questions of a similar