Henry Coddington

“ Luminous undulations ought to produce a similar effect when the medium in which they are excited is terminated by a body in which the density of the ethereal fluid is different; that is to say, there must be produced a reflected wave and one transmitted; which is, in fact, what we call reflexion and refraction. In this system, the intensities of rays of light must be measured by the vis viva of the fluid in motion, that is, by the product of the density of the fluid by the square of the proper velocity of its particles. ”

“ If the mode of observation which we have applied to smooth glass plates be universally employed, it may serve to show that polarization when complete is always a modification exactly of the same kind, for all substances: for when a beam of light has been once polarized, it will equally pass through all substances, with the exception mentioned above, provided each be presented to it under its proper angle, and whatever be the nature of the first or second substance employed, the variation of intensity in the light after the second reflexion is always subject to the same laws. ”

“ And if rays are admitted to all points on the surface of the lens, the points v, i, &c. will be the vertices of so many cones of light of the different colours.
As all the rays do not converge to one point, it is important to know at least where they approach most nearly to it, or where they are all collected in the least space, and how great that space is, which is, in technical language, to require the center and diameter of the least circle of chromatic aberration or dispersion. ”

“ Since reflexion and refraction, even of the ordinary kind, modify the polarization of light, we may expect to find this effect produced when rays of light are made to pass through media of regularly varying density. It is accordingly found that all transparent bodies which are sufficiently elastic to admit of different positions of their particles round a given state of equilibrium, as glass, crystals, animal jellies, horn, &c. produce phænomena of polarization when they are compressed or expanded, or made unequally dense by being considerably heated and then cooled suddenly and unequally. ”

“ It is evident then, that if the eye glass be a powerful lens, which it always is in practice, the final image subtends a much larger angle at the eye which is placed close to the eye glass, than the object does, and it is brighter, for the eye receives in the one case all the light which falls on the object glass, in the other, only as much of that coming from the object as can pass through the pupil of the eye. ”

“ When a pencil of light naturally emanating from a luminous body, passes through a single-axed crystal, and is divided into two pencils having different directions, each of these pencils is polarized in one single direction; the ordinary one in the plane passing through its direction and a line parallel to the axis of the crystal, the extraordinary one perpendicularly to a plane similarly situated with respect to its direction. Either of these rays, when received on a plate of glass after its emergence, shows all the characters of polarization that we have described. ”

“ This medium, if it does exist, must fill all the expanse of the heavens, since it is through this expanse that the light of the stars comes to our eyes: it must also be extremely elastic, since the transmission of light takes place with such extraordinary velocity; and at the same time its density must be almost infinitely small, since the most exact discussion of ancient and modern astronomical observations, does not indicate the least trace of resistance in the planetary motions. ”

“ In the case of a plane mirror we have seen, that when any pencil of rays falls on it, the reflected light proceeds from a point at the same distance behind the mirror, as the focus of incident rays is before it. ”

“ The effect of the camera obscura may be produced without a lens, merely by admitting light through a small aperture in a shutter into a darkened room, for the rays from each point of an external object being allowed to illuminate only a very small surface, will produce a tolerably distinct image without being made to converge, provided this image be received on a white surface, placed near the hole in the shutter. ”

“ Among all the positions that may be given to the crystal, resting always on the same face, there is one which deserves particularly to be remarked, because the extraordinary refraction takes place, like the ordinary, in the plane of emergence. ”

“ Many other crystallized substances, very different from the Iceland spar, exhibit like it a certain single line or axis, round which their double refraction is exerted symmetrically, being insensible for rays parallel to that axis, and increasing with their inclination to it, so as to be strongest for those which are at right angles to the axis. Crystals thus constituted are called crystals with one axis. For instance, quartz, commonly called rock crystal, has an axis parallel to the edges of the hexahedral prism, under the form of which it is generally found. ”

“ In most cases in which the reflexion is made by a concave mirror, the rays proceeding from the various points of an object are made to converge, and actually meet in points constituting the image, and afterwards diverge from those points, as they would from a real surface placed there, so as to make it visible. ”

“ Upon the whole we may collect that if a small luminous body be placed before a spherical concave mirror, at some distance from it, the distance of the focus will always be something more than half the radius of the surface, which is its accurate value for the light of the Sun, the rays of which are considered as parallel ”

“ These laws were, however, still complicated in consequence of the unequal refrangibilities of the different rays, by which the rings were illuminated. To reduce the phænomenon to its greatest simplicity, Newton formed rings with simple light, by looking, in a dark room, at a white paper, which received in turns all the simple colours of the prismatic spectrum. This paper thus enlightened, and seen by reflexion on the thin laminæ, became like a kind of sky, coloured by that tint alone, which was thrown on it. ”

“ We have hitherto considered light as something, whether substance or affection of substance, of one single kind. The fact is, however, that light, as it comes to us from the Sun, is not homogeneous, but consists of various kinds, mixed together in different proportions, which being refrangible in different degrees by the same medium, may be exhibited very clearly by causing a Sunbeam to pass through a prism and fall on a sheet of paper placed in a proper position or the wall of a room. ”

“ It will readily occur to the reader, that when the lower part of the atmosphere is very much rarified, so that its density diminishes rapidly to the ground, it is possible for a ray of light tending downwards, to be reflected, as it is in glass, or water, when the angle of incidence exceeds a certain value. ”

“ For extraordinary refraction, we have not the advantage of being able to define the origin of the molecular force, nor the manner in which it emanates individually from each particle of the crystal; for what we have said about accounting for the phænomena by the supposition of attractive and repulsive forces emanating from the axes is only the indication of a complicated result, and not the expression of a molecular action. ”

“ The rays of light are straight lines. The angles of incidence and reflexion are in the same plane and equal. The angles of incidence and refraction are in the same plane, and their sines bear an invariable ratio to one another for the same medium. ”

“ Descartes, and after him Huyghens, and a great number of natural philosophers, have supposed that the sensation of light was produced in us by undulations excited in a very elastic medium, and propagated to our eye, which they affect in the same manner as undulations excited in the proper medium of the air, and propagated to the ear, produce in it the sensation of sound. ”

“ The polarization of light is a property discovered by Malus, which consists in certain affections that the rays of light assume on being reflected by polished surfaces, or refracted by these same surfaces, or transmitted through substances possessing double refraction. ”

“ When a very great power is required, it is not uncommon to use a minute spherule of glass, of water, or of colourless varnish, stuck in a needle-hole in a plate of metal, which should be ground hollow on both sides, so as to be as thin as possible, where the aperture is made. The distance of the principal focus of a sphere from its surface being only half the radius, the magnifying power of such an apparatus is very great. ”

“ In whatever manner, and under whatever circumstances these rings are formed, the succession of their colours from the central dark spot is invariably the same; the only difference perceptible is in their brightness, which varies with the refracting power of the lamina, and in their form, which depends on the law by which the thickness of the lamina is regulated in different parts. In fact, for any one substance, the colour reflected at any point depends on the thickness of the lamina, and the incidence under which the reflexion takes place. ”

“ Hence, the image of any object situated obliquely, with respect to the mirror and the eye, will be more or less confused, besides being situated in a different place from that which it would occupy, in the case of a plain metallic speculum without glass. ”

“ These are the only cases in which the caustic to a spherical reflector is a known curve; we may, however, without much difficulty, make out the kind of figure that it assumes in other cases, though the investigation of the equation is very tedious.
When the radiant point is at a finite distance without the circle, the form of the caustic is naturally intermediate to those already found. It is represented in Fig. 27. The caustic touches the circle at the points C, c where tangents from Q meet it, and it has a cusp at V, the focus of the principal reflected rays. ”

“ What is the focal length of a lens composed of water contained between two meniscus-shaped watch-glasses, the radii of the surfaces being 5 and 7 inches, and the thickness supposed inconsiderable.
18. Supposing a diamond sphere to be just inclosed in a cube of glass, what would be the focus of rays incident perpendicularly at one of the points where the surfaces touch?
19. The caustic by refraction of a plane surface, for rays diverging from a point, is the evolute of an ellipse or an hyperbola, according as the passage is into a denser or a rarer medium. ”

“ It is, however, to be observed, that the small reflector is usually of very confined dimensions, and when the object speculum is well ground, it is found that the lateral rays converge quite sufficiently well to make a distinct image.
To find the magnifying power, we must compare the angle subtended by the second image at the eye glass with that of the object or first image at the center of the great mirror: but since the focal length of the mirror is half its radius, this is the same thing as the angle of the first image seen from the mirror. ”

“ Now this is just the case with the atmosphere; for the air being an elastic fluid, is compressed and condensed by its own weight, so that its density increases continually from its extreme bounds to the surface of the earth; and, in consequence of this, all distant objects, whether celestial or terrestrial, except those immediately over-head, are seen by curved rays, and consequently referred by the spectator to places which they do not really occupy. ”

“ If light is to be considered as a material substance, the refraction of its rays must be produced by attractive forces, exerted by the particles of other bodies on the luminous molecules, forces which can be sensible only at very minute distances, and which are therefore quite analogous to those which are exerted in chemical affinities. It follows, that when particles of light are at a sensible distance from a refracting body, the effect they experience from it is quite inappreciable, so that their natural rectilinear direction is not altered ”

“ Here a convex lens produces a distant, and therefore magnified image of a near transparent object, which is strongly enlightened by a lamp placed behind it; a concave mirror is generally put behind the lamp to throw as much of the light as possible on the object, which is some group of figures painted on a glass slide, and is inverted so that the image may be cast in an erect position, on a wall or a white sheet, at a proper distance. ”

“ One may even see whether the ordinary refraction follows, in all cases, the law of the proportionality of the sines.
It is necesary to make the edge of the crystal prism as sharp as possible, in order that the corrections made for its thickness be inconsiderable. In fact the best way of making the observation, when it can be done, is to let the rays pass actually through the edge, for then the two refracted pencils have but an infinitely small space to pass through, before they emerge together. ”

“ Young has with equal success applied the principle of interferences to the explanation of the coloured rings, both reflected and transmitted, of thin plates. When such a plate is seen by reflexion, the light coming from the first surface to the eye interferes with that from the second; this interference either does or does not produce the sensation of light, according as the different distances that the rays have to pass over, place them in similar or opposite phases of their undulations ”