when of glass or platina. 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 colored fringes or destroy one another in the obscure intervals. The colored 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 colored rings are produced by the interference of light.

It may be concluded, that material substances derive their colors 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 colored 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 colored 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 extin-guished 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

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become thus comparatively stagnant. The heating power of the solar rays gives a prima 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 nature may perhaps admit of answer in a more advanced state of our knowledge; but at present there is none obvious. It is not without reason, therefore, that the question What becomes of light?' which appears to have been agitated among the photologists of the last century, has been regarded as one of considerable importance as well as obscurity by the corpuscular philosophers. On the other hand, the answer to this question, afforded by the undulatory theory of light, is simple and distinct. The question, 'What becomes of light?' merges in the more general one, What becomes of motion? And the answer, on dynamical principles, is, that it continues forever. No motion is, strictly speaking, annihilated; but it may be divided, and the divided parts made to oppose and, in effect, destroy one another. A body struck, however perfectly elastic, vibrates for a time, and then appears to sink into its original repose. But this apparent rest (even abstracting from the inquiry that part of the motion which may be conveyed away by the ambient air) is nothing else than a state of subdivided and mutually destroying motion, in which every molecule continues to be agitated by an indefinite multitude of internally reflected waves, propagated through it in every possible direction, from every point in its surface on which they successively impinge. The superposition of such waves will, it is easily seen, at length operate their mutual destruction, which will be the more complete the more irregular the figure of the body, and the greater the number of internal reflections." Thus Sir John Herschel, by referring the absorption of light to the subdivision and mutual destruction of the vibrations of ether in the interior of

bodies, brings another class of phenomena under the laws of the undulatory theory.

The ethereal medium pervading space is supposed to penetrate all material substances, occupying the interstices between their molecules; but in the interior of refracting media it exists in a state of less elasticity compared with its density in vacuo; and the more refractive the medium, the less the elasticity of the ether within it. Hence the waves of light are transmitted with less velocity in such media as glass and water than in the external ether. As soon as a ray of light reaches the surface of a diaphanous reflecting substance, for example a plate of glass, it communicates its undulations to the ether next in contact with the surface, which thus becomes a new center of motion, and two hemispherical waves are propagated from each point of this surface; one of which proceeds forward into the interior of the glass, with a less velocity than the incident waves; and the other is transmitted back into the air, with a velocity equal to that with which it came (N. 198). Thus when refracted, the light moves with a different velocity without and within the glass; when reflected, the ray comes and goes with the same velocity. The particles of ether without the glass, which communicate their motions to the particles of the dense and less elastic ether within it, are analogous to small elastic balls striking large ones; for some of the motion will be communicated to the large balls, and the small ones will be reflected. The first would cause the refracted wave; and the last the reflected. Conversely, when the light passes from glass to air, the action is similar to large balls striking small ones. The small balls receive a motion which would cause the refracted ray, and the part of the motion retained by the large ones would occasion the reflected wave; so that when light passes through a plate of glass or of any other medium differing in density from the air, there is a reflection at both surfaces; but this difference exists between the two reflections, that one is caused by a vibration in the same direction with that of the incident ray, and the other by a vibration in the opposite direction. A single wave of air or ether would not produce the

sensation of sound or light. In order to excite vision, the vibrations of the molecules of ether must be regular, periodical, and very often repeated; and as the ear continues to be agitated for a short time after the impulse by which alone a sound becomes continuous, so also the fibres of the retina, according to M. d'Arcet, continue to vibrate for about the eighth part of a second, after the exciting cause has ceased. Every one must have observed, when a strong impression is made by a bright light, that an object remains visible for a short time after shutting the eyes, which is supposed to be in consequence of the continued vibrations of the fibres of the retina. Occasionally the retina becomes insensible to feebly illuminated objects when continuously presented. If the eye be turned aside for a moment, the object becomes again visible. It is probably on this account that the owl makes so peculiar a motion with its head when looking at objects in the twilight. It is quite possible that many vibrations may be excited in the ethereal medium incapable of producing undulations in the fibres of the human retina, which yet have a powerful effect on those of other animals or of insects. Such may receive luminous impressions of which we are totally unconscious, and at the same time they may be insensible to the light and colors which affect our eyes; their perceptions beginning where ours end.

SECTION XXI.

Polarization of Light-Defined-Polarization by Refraction-Properties of the Tourmaline-Double Refraction-All doubly Refracted Light is Polarized--Properties of Iceland Spar-Tourmaline absorbs one of the two Refracted Rays-Undulations of Natural Light-Undulations of Polarized Light-The Optic Axes of Crystals-M. Fresnel's Discoveries on the Rays passing along the Optic Axis-Polarization by Reflection.

IN giving a sketch of the constitution of light, it is impossible to omit the extraordinary property of its polarization, "the phenomena of which," Sir John Herschel says, "are so singular and various, that to one who has only studied the common branches of physical optics, it is like entering into a new world, so splendid

as to render it one of the most delightful branches of experimental inquiry, and so ferten the views i wys open of the constitution of natural bodies, and the minuter mechanism of the universe, as to place it in the very first rank of the physico-mathematical sciences, which it maintains by the rigorous application of geometrical reasoning its nature admits and requires.

Light is said to be polarized, which, by being once reflected or refracted, is rendered incapable of being again reflected or refracted at certain angies. In general, when a ray of light is reflected from a pane of plateglass, or any other substance, it may be reflected a second time from another surface, and it will also pass freely through transparent bodies. But if a ray of light be reflected from a pane of plate-glass at an angle of 57°, it is rendered totally incapable of reflection at the surface of another pane of glass in certain definite positions, but it will be completely reflected by the second pane in other positions. It hkewise loses the property of penetrating transparent bodies in particular positions, while it is freely transmitted by them in others. Light so modified as to be incapable of reflection and transmission in certain directions, is said to be polarized. This name was originally adopted from an imaginary analogy in the arrangement of the particles of light on the corpuscular doctrine to the poles of a magnet, and is still retained in the undulatory theory.

Light may be polarized by reflection from any polished surface, and the same property is also imparted by refraction. It is proposed to explain these methods of polarizing light, to give a short account of its most remarkable properties, and to endeavor to describe a few of the splendid phenomena it exhibits.

If a brown tourmaline, which is a mineral generally crystalized in the form of a long prism, be cut longitudinally, that is, parallel to the axis of the prism, into plates about the thirtieth of an inch in thickness, and the surfaces polished, luminous objects may be seen through them, as through plates of colored glass. The axis of each plate is in its longitudinal section parallel to the axis of the prism whence it was cut (N. 199). If one of these plates be held perpendicularly between