product of wisdom, and to rear up an impotent agency into a work which Almighty Power could alone perfect: it is, in fact, to sink Omnipotence and Omniscience into the mere revolutions of ages, or senseless progressions and operations of dead matter; which, instead of order, beauty, and stability, fail not, on all occasions, to produce incongruity, disorder, weakness, and ruin. Infinite power and infinite wisdom are equally observable throughout creation; and no where are they more observable than in the crust of the sphere assigned by the Great Creator to us, for our habitation and solace. Here then, while we survey the work, let us look up to and adore the Creator, whose presence and activity were not more needful during the creation, than they are during the ages of His divine providence over all that He hath called into existence : "For of Him, and through Him, and to Him, are all things: to whom be glory for ever.' Amen.

King Square, Nov 20, 1831.

W. COLDWELL.

SOME OBSERVATIONS ON THE NATURE OF LIGHT AND COLOURS. (Concluded from p. 90.)

THOUGH no body in nature be perfectly, all are to a certain degree, transparent. One of the densest of metals, gold, may actually be beaten so thin as to allow light to pass through it; and, that it passes through the substance of the metal, not through cracks or holes too small to be detected by the eye, is evident from the colour of the transmitted light, which is green, even when the incident light is white. All coloured bodies, however deep their hues, and, however seemingly opaque, must necessarily be rendered visible by rays which have entered their substance; for, if reflected at their surfaces, they would all appear white alike. Were the colours of bodies strictly superficial, no variation in thickness could affect their hue; but, so far is this from being the case, that all coloured bodies, however intense their tint, become paler by diminution of

their thickness.

This gradual diminution in the intensity of a transmitted ray, in its progress through imperfectly transparent media, is termed its absorption. It is never found to affect equally rays of all colours, some being always absorbed in preference to others; and it is on this preference that the colours of all such media, as seen by transmitted light, depend. A white ray transmitted through a perfectly transparent medium,

ought to contain at its emergence the same proportional quantity of all the coloured rays, because the part reflected at its anterior and posterior surfaces is colourless; but, in point of fact, such perfect want of colour in the transmitted beam is never observed. Media, therefore, are unequally transparent for the differently coloured rays. Each ray of the spectrum has, for every different medium in nature, its own peculiar index of transparency, just as the index of refraction differs for different rays and different media.

The following simple experiment shews, in a striking manner, the different absorptive power of one and the same medium on differently-coloured rays. Look through a plain piece of blue glass, (such as sugarbasins, and finger-glasses are often made of,) at the image of any narrow line of light, (as the crack in a window-shutter of a darkened room,) refracted through a prism, whose edge is parallel to the line, and placed in its situation of minimum deviation. If the glass be extremely thin, all the colours are seen; but if of moderate thickness, (as one twentieth part of an inch,) the spectrum will put on a very singular and striking appearance. It will appear composed of several detached portions, separated by broad and perfectly black intervals, the rays which correspond to those points in the perfect spectrum being entirely extinguished. If a less thickness be employed, the intervals, instead of being entirely dark, are feebly and irregularly illuminated, some parts of them being less enfeebled than others. If the thickness, on the other hand, be increased, the black spaces become broader, till at length all the colours intermediate between the extreme red and extreme violet, are totally destroyed.

The simplest hypothesis we can form of the extinction of a beam of homogeneous light, in passing through a homogeneous medium, is, that for every thickness of the medium passed through, an equal aliquot part of the rays, which up to that depth had escaped absorption, is extinguished. Thus, if one thousand red rays fall on and enter into a certain green glass, and if one hundred be extinguished in traversing the first tenth of an inch, there will remain nine hundred which have penetrated so far; and of these one-tenth, or ninety, will be extinguished in the next tenth of an inch, leaving eight hundred and ten, out of which, again, a tenth, or eighty-one, will be extinguished in traversing the third-tenth, leaving seven hundred and twenty-nine, and so on.

When a ray of white or solar light falls obliquely on the surface of a refracting medium, it is not refracted entirely in one direction, but undergoes a separation into several rays, and is dispersed over an angle more or less considerable, according to the nature of the medium, and the obliquity of incidence. The several rays of which the dispersed beam consists, are found to differ essentially from each other, and from the incident beam, in a most important physical character. They are of different colours. The light of the sun is white, and if a sun-beam be admitted into a dark ened room through a small round hole in the window-shutter, and be received directly on a piece of paper, it makes on it a round white spot, which will be larger as the paper is further removed. To shew the separation or dispersion of the rays, take a triangular prism of good flint glass, and place it in the beam with one of its angles downwards, so that the beam may fall on one of its sides obliquely. The beam will then be refracted, and turned out of its course, and thrown upwards, and may be received on a screen properly placed. But on this screen there will no longer be seen a round white spot, but a long streak, or, as it is called in optics, a spectrum of most vivid colours. The tint of the lower or least refracted extremity will be a brilliant red, then an orange, afterwards a pale straw yellow, succeeded by a pure and very intense green, which passes to a blue, at first greenish, but, as the distance increases, deepening into the purest indigo, and, as the intensity of the illumination diminishes fading into a pale violet.

If the screen on which the spectrum be received have a small hole in it, only large enough to allow a very narrow portion of the spectrum to pass, and this portion of the beam be received on another screen, placed at some distance behind it, it will there form a spot of the very same colour as that portion of the spectrum allowed to pass. Thus, if the hole be placed in the red part of the spectrum, the spot will be red; if in the green, green; and in the blue, blue. If the eye be placed so as to see through this small hole, an image of the sun will be beheld, of dazzling brightness, not, as usually, white, but of the colour of that portion of the spectrum which goes to form the spot on the screen.

If, instead of receiving the ray transmitted through the small hole in the first screen on a second screen immediately behind it, it be intercepted by another prism, it will be refracted and bent from its course, as in the first instance; and, after this second 2D. SERIES, NO. 15.-VOL. II.

refraction, may be received on a third

screen.

But it is now observed to be no longer separated into a coloured spectrum, like the original one of which it formed a part. A single spot only is seen on the screen, the colour of which is uniform, and precisely the same as that portion of the spectrum from which it is taken. It appears, then, that the ray which goes to form any single point of the spectrum, is not only independent of all the rest, but, having been once insulated from them, is no longer capable of further separation into different colours, by a second refraction.

From the above-mentioned simple expements, the following properties of light may be deduced.

1. A beam of white or solar light consists of a great and almost infinite variety of rays, differing from each other in colour and refrangibility.

2. White light may be decomposed, analyzed, or separated into its elementary coloured rays by refraction. The act of such separation is called the dispersion of the coloured rays.

3. Each elementary ray, once separated and insulated from the rest, is incapable of further decomposition or analysis by the same means. For we may place a third, and a fourth prism in the way of the twice refracted ray, and refract in any way, or in any plane; it remains undispersed, and preserves its colour quite unaltered.

4. The dispersion of the coloured rays takes place in the plane of the refraction; for it is found that the spectrum is always elongated in this plane.

That the term analysis or decomposition, is correct as applied to the separation of a beam of white light into coloured rays, may be proved by the following experiment, in which, by a synthesis or joining together of the elementary rays, white light is again produced.

If a small circular beam of solar light be passed through a prism, and the dispersed coloured rays received in a lens at some distance, and transmitted to a white screen, the whole spectrum, instead of being coloured, will be re-united in a spot of white light.

That the re-union of all the coloured rays is necessary to produce whiteness, may be shewn by intercepting a portion of the spectrum before it falls on the lens. Thus, if the violet ray be intercepted, the white spot will acquire a tinge of yellow; if the blue and green be successively stopped, the yellow tinge will grow more and more ruddy, and pass through orange to scarlet and blood red. If, on the other hand, the 139.-VOL. XIV.

R

red end of the spectrum be stopped, and more and more of the less refrangible portion thus successively abstracted from the beam, the white will pass first into pale, and then to vivid green, blue green, blue, and finally into violet. If the middle portion of the spectrum be intercepted, the remaining rays, concentrated, produce various shades of purple, crimson, or plum-colour, according to the portion by which it is thus rendered deficient from white light; and by varying the intercepted rays, any variety of colours may be produced; nor is there any shade of colour in nature which may · not thus be exactly imitated, with a brilliancy and richness surpassing that of any artificial colouring.

According to the Newtonian doctrine of the origin of colours, and every phenome non in optics conspires to prove the truth of it

"The colours of natural bodies are not qualities inherent in the bodies themselves, by which they immediately affect our sense, but are mere consequences of that peculiar disposition of the particles of each body, by which it is enabled more copi ously to reflect the ray of one particular colour, and to transmit, or stifle, or, as it is called in optics, absorb the others."

Perhaps the most direct and satis. factory proof of the truth of this doctrine is to be found in the simple fact, that every body indifferently, whatever be its colour in white light, when exposed in the prismatic spectrum, appears of the colour appropriate to that part of the spectrum in which it is placed; but that its tint is incomparably more vivid and full, when laid in a ray of a tint analogous to its hue in a white light, than in any other. For example, vermilion placed in the red rays appears of the most vivid red; in the orange, orange; in the yellow, yellow, but less bright. In the green rays, it is green; but from the great inaptitude of vermilion to reflect green light, it appears dark and dull; still more so in the blue; and in the indigo and violet it is almost completely black. On the other hand, a piece of dark blue paper, or Prussian blue, in the indigo rays has an extraordinary richness and depth of colour. In the green its hue is green, but much less intense; while in the red rays, it is almost entirely black.

In the above experiments, to make the analysis complete, the beam of light to be analyzed must be very small, and the prism as free from striæ or veins as possible, but as it is diffiult to procure prisms free from these imperfections, the best way is to

employ hollow prisms filled with water, or some of the more dispersive oils. If these are not at hand, the inconvenience may be diminished by transmitting the ray as near the edge of the prism as possible, so as to lessen the quantity of the material it has to pass through, and therefore the chance of encountering striæ in its passage.

It has been observed, that there are many crystallized minerals, especially the tourmaline, which when cut into parallel plates are sufficiently transparent, and let pass abundance of light with perfect regularity, nevertheless, the light at its emergence is found to have acquired a peculiar modification, which has been termed polarity, or polarization. The difference between a polarized and an ordinary ray of light can hardly be more readily explained than by assimilating the latter to a cylindrical, and the former to a four-sided prismatic rod, or flat ruler; that is to say, the polarized rays seems to have acquired sides, and to be rendered incapable of passing through certain media permeable to it in its original or unpolarized state, as a broad flat ruler will not pass through the bars of a narrow grating, if presented to it crossways.

The following experiment will exemplify the thing clearly.

The tourmaline, which is a species of schorl, crystallizes in long prisms, whose primitive form is the obtuse rhomboid, having its axis parallel to the axis of the prism. The lateral faces of these prisms are frequently so numerous as to give them an approach to a cylindrical or cylindroidal form. Now, if one of these crystals be taken and slit (by the aid of a lapidary's wheel) into plates parallel to the axis of the prism of a moderate and uniform thickness, (about one twentieth of an inch,) which must be well polished, luminous objects may be seen through them, as through plates of coloured glass. Let one of these plates be interposed perpendicularly between the eye and a candle, the latter will be seen with equal distinctness in every position of the axis of the plate with respect to the horizon; and if the plate be turned round on its own plane, no change will be perceived in the image of the candle. Now, holding this first plate in a fixed position, (with its axis vertical, for instance,) let a second be interposed between it and the eye, and turned round slowly in its own plane, and a very remarkable phenomenon will be seen. The candle will appear and disappear alternately at every quarter revolution of the plate, passing through all gradations of brightness, from a maximum down to

a total, or almost total, evanescence, and then increasing again by the same degrees as it diminished before. Now, it is evident that the light which has passed through the first plate, has acquired, in so doing, a property totally distinct from those of the original light of the candle. The latter would have penetrated the second plate equally well in all its positions; the former is incapable altogether of penetrating it in some positions, while in others it passes through readily, and these positions correspond to certain sides which the ray has acquired, and which are parallel and perpendicular respectively to the axis of the first plate. Moreover, these sides, once acquired, are retained by the ray in all its future course, (provided it be not again otherwise modified by contact with other bodies,) for it matters not how great the distance between the two plates, whether they be in contact, or many inches, feet, or yards asunder, not the least variation is perceived in the phenomenon in question. If the position of the first plate be shifted, the sides of the transmitted ray shift with it, through an equal angle, and the second will no longer extinguish it in the position it at first did, but must be brought into a position removed therefrom by an angle equal to that through which the first plate has been made to revolve.

But it is not only by such means that the polarization of a pencil of light may be effected, nor is this the only character which distinguishes polarized from ordinary light. It may be as well, therefore, briefly to mention the principal means by which the polarization of light may be performed, and the characters which are invariably found to co-exist in a ray when polarized.

The polarization of light may be effected, 1. By reflexion, at a proper angle from the surfaces of transparent media.

2. By transmission through a regularly crystallized medium, possessed of the property of double refraction.

3. By transmission through transparent, uncrystallized plates, in sufficient numbers, and at proper angles.

4. By transmission through a variety of bodies which have an approach to a laminated structure, and an imperfect state of crystallization, such as agate, mother-ofpearl, &c.

The characters which are invariably found to co-exist in a polarized ray, and by which it may be most easily recognized as polarized, are,

1. Incapability of being transmitted by a plate of tourmaline, as above described, when incident perpendicularly on it, in

certain positions of the plate; and ready transmission in others, at right angles to the former.

2. Incapability of being reflected by polished transparent media, at certain angles of incidence, and in certain positions of the plane of incidence.

3. Incapability of undergoing division into two equal pencils by double refrac tion, in positions of the doubly-refracting bodies, in which a ray of ordinary light would be so divided.

We have not room to describe the experiments by means of which the phenomena, above alluded to, are performed and explained; but it may be necessary to remark, that the characters of polarized light are all of the negative kind, and consist in denying to it properties which ordinary light possesses, and that they are such as affect the intensity of the ray, not its direction. Thus, the direction which a polarized ray will take, under any circumstances of the action of media, is never different from what an unpolarized ray might take, and from what a portion of it, at least, actually does. For instance, when an unpolarized. ray is separated by double refraction into two equal pencils, a polarized ray will be divided into two unequal ones, one of which may even be altogether evanescent, but their directions are precisely the same as those of the pencils into which the unpolarized ray is divided. Hence, it may be laid down as a general principle, that the direction taken by a polarized ray, or by the parts into which it may be divided by any reflexions, refractions, or other modifying causes, may always be determined by the same rules as apply to unpolarized light; but that the relative intensities of these portions differ from those of similar portions of unpolarized light, according to certain laws, which it is the business of the philosopher to ascertain.

From the foregoing observations and experiments, the following facts, relative to the nature and properties of light, may be considered as established.

1. Light has never been found collected in separate masses, but variously manifests its existence in several bodies.

2. Light possesses the property of exciting in us the sensation of vision, by moving from an illuminated object to the eye.

3. The motion of light is progressive, being known to occupy about seven and a half minutes in moving from the sun to the earth.

4. Its progress may be stopped by the interposition of an opaque body, and the