530 LIGHT tion of copiously, as by the emission and re-action of its light, and the ivfk-xion-i and refractions of its rays within its pores, to grow still hotter, till it comes to a certain period of heat, such as is that of the sun ? And are not the sun and fixed stars great earths vehe mently hot ; whose heat is conserved by the greatness of the bodies, and the mutual action and re-action between them, and the light which they emit ; and whose parts are kept from fuming away, not only by their Fixity, but also by the vast weight and density of the atmospheres incumbent upon them, and very strongly compressing them, and condensing the vapours and exhalations which arise from them ? . . . . And the same great weight may condense those vapours and exhalations, as soon as they shall at any time begin to ascend from the sun, and make them presently fall back again into him ; and by that action increase his heat, much after the manner that in our earth the air increases the heat of a culinary fire. And the same weight may hinder the globe of the sun from being diminished, unless by the emission of light, and a very small quantity of vapours and exhalations." Mecinri- THEORIES OF PROPAGATION OF LIGHT. We may begin ism of by assuming that the sensation of light is due to a mQchanicai ac tion on the retina (see EYE). Now such . , . , x i i a mechanical action must have a mechanical cause, and, as far as we can judge with our present knowledge, tli3 latter must consist of impacts on the retina, due to moving matter. This matter may have travelled all the way from the source of light, or it may have been set in motion in the eye by a disturbance (analogous to a wave) which has travelled from the source. What is trans ferred, or what moves, is a quite independent question, Light must, as far as we can conceive, consist in the motion of particles of some kind from external objects to the eye, or in the propagation of some disturbance or wave- motion in an as yet unknown medium. Though it has been proved, as we will presently show, that some of the consequences of the first supposition are entirely incon sistent with observed facts, the nature of the propagation of the supposed luminous particles is still a very interest ing study, and indeed many of the fundamental propositions in optics follow more easily from this hypothesis than from the other. We will therefore not at present dismiss this hypothesis, but will refer freely to it now and then, until its truth is shown to be inconsistent with experiment. Cor- This view, associated with the names of Newton, Laplace, puscular anc | Biot, is known as the corpuscular theory of light. A eoi ^ r formidable objection to it, in limine, will be easily seen to be furnished by the velocity of light. Since every point of every visible body must (on this theory) send such corpuscles to the eye, moving as we shall find at a rate of nearly 200,000 miles per second, their masses must be inconceivably minute iu order that their united momentum may not amount to something comparable with that of a cannon shot, a supposition of course utterly destructive of all belief in the hypothesis. But, as we shall see, there are still higher grounds of objection, and such as no mere smallness of mass or size of each corpuscle can explain away. Unilula- The rival theory labours under considerable clisadvan- tory tages, inasmuch as the theory of wave-propagation is very oiy- much more obscure and difficult than that of the motion of free particles; but the student, who lias mastered the fundamental difficulties of sound (see ACOUSTICS), which presents a fair although not an exact analogy, will find it comparatively easy to obtain a clear conception of the lundamental principles of the explanation offered by the undulatory theory of light. The difference between these two theories of light may be illustrated by contrasting wind moving at the rate of 1100 feet per second (an inconceivably violent hurricane) and sound, gentle or violent, moving at precisely the same rate yet how different in its effects ! DIVISION OF THE SUBJECT. Optics, or the science of Light, is usually divided into two parts. A simple illus tration of the nature of this division will be found in Geo metrical and physical optics. Proposed order of treat ment. the different conditions of fluid equilibrium according as we do not or do introduce the idea of action between the fluid and the containing vessel (CAPILLARY ACTION, q.v.). In the first or hypothetical case it is known that the free surface must be horizontal, and that all its separate parts must lie in the same plane ; in the second, i.e., the actual, case we find molecular action modi fying these results, sometimes indeed to a very large extent, so that no part of the free surface is plane, and no two portions of it are at the same level. So in what is called GEOMETRICAL OPTICS it is assumed from experiment that light moves in straight lines in air, while PHYSICAL OPTICS, or the undulafeory theory, agrees with experiment in showing that under certain circumstances a ray of light bends round an obstacle. But as, in obtaining the main facts of fluid equilibrium, capillary forces may be neglected, so, for the explanation of the ordinary phenomena of light, even with accuracy sufficient for the construction of the very finest telescopes and microscopes, it suffices that Geometrical Optics, based on laws nearly verified by ex periment, be followed out to its consequences. The residual phenomena then came in to be treated by the undulatory theory. Pouillet divides the subject, in con sequence of this distinction, into two parts, viz., (1) that in which we deal with the direction only of the rays, and (2) that in which we deal with the physical properties of the rays themselves. In this order we will consider the subject, giving the explanations of the approximate experimental laws of Geometrical Optics, as we reach them, in the language of either theory. But before we come to the residual phenomena we shall have found that the corpuscular theory must be rejected, and we will therefore give, without detail, the principles of the undulatory explanation, which will be fully discussed in a special article. GEOMETRICAL OPTICS. Rectilinear Propagation of Light. It is approximately true that, in any homogeneous medium, light moves in straight lines. If an opaque body be placed anywhere in the straight line between the eye and an object, the object is concealed. Through a long straight tube no objects can be seen but those situated in the direction of its axis produced. This is so fundamental a fact, or it is so evident a result of experience, that it is the foundation of every process which involves the direction in space of one object as regards another, whether it be for the aiming with a rifle, the pointing of a telescope, or for the delicate observations of a geodetic survey. But we must carefully observe the re strictions under which the statement is made. Not merely is it said to be only approximately true, but it is so only in a homogeneous medium. To both of these restrictions we will revert later. (a) On this is founded the geometrical theory of shadoios, Shadows a subject of some importance, especially as regards eclipses. In this application the results may be considered as absolutely true, though, as we shall see in a subsequent page, the statement is liable in certain delicate cases to somewhat startling exceptions. When an opaque body is placed between a screen and a luminous point, it casts a shadow on the screen. (The sun s image formed by a lens or burning glass of short focus is our best mode of attempting to realize the conception of a luminous point; but a fair approximation may be made by piercing a very small needle-hole in a large plate of thin metal, and placing it close to any bright flame or incandescent body.) The outline of the shadow is, of course, to be found by drawing straight lines from the luminous point so as to touch the
