stices be the same, because the edges of the interstices absorb part of the light.

The moment light comes in contact with any black pigment, or black material, that moment its particles are decomposed. They are either disunited, and enter other compounds, or else return to their latent state again. Light undergoes some change by which its character or quality of luminousness disappears; whatever the nature of the black principle may be, certain it is that it has the remarkable property of disuniting the particles that cause luminousness; and in proportion to the blackness of the colouring matter, will light, as luminousness, be decomposed by it.

Light, therefore, is always resolved into its latent state the instant it encounters the black principle. There is, therefore, an antagonist to light as well as to heat, and we no more know the nature of blackness than we do of luminousness. We cannot say that blackness is the lowest degree of luminousness; for, in the midst of the most intense light, if a black pigment intervene, this light is suddenly extinguished. These two principles are opposing forces, and keep each other in check.

Philosophers have not yet sufficiently analysed the effects of these powers; their principal aim has been to dispute whether light be a material substance, or only undulations; as to its antagonist, blackness, that has never been considered an independent power at all. Why is it that both cold and blackness have stood so low in the scale, when their effects are quite as conspicuous as those of heat and light?

When we perceive a great accumulation of the black principle, nothing is conveyed from it to our vision but blackness; when there is a great accumulation of light, nothing is conveyed to our vision but a dense mass of light. A dense mass of light obstructs vision as much as a dense mass of blackness. If a quantity of light fall from a hole in the window-shutter on a coloured material, this light, being opaque in consequence of its density, prevents our seeing the colours through it. It has been insisted on, for upwards of two hundred years, that light decomposes colours, or rather resolves them all into one, and that one is pure white. The action of light on certain colouring matter will, eventually, destroy the freshness and brightness of the lustre, but not the entire colours themselves; and this change will always be perceived; even a strong black colour will acquire a rusty tint. But although we do not see the colours on a part of a bright hearth rug when a dense mass of light falls on it, yet, the moment the body of light is withdrawn, the colours are as bright as ever; if these colours had been changed to one of pure white by the action of the sun's rays, they would remain changed on that spot; but this is not the case.

Whether the colouring principle be in light-which we doubt -or in the material, or whether it originate on the cerebral organs of vision, is a question not yet solved; but, wherever its seat may be, it is evident that its destruction does not arise alone from contact with luminous rays of light-its sudden decomposition we mean. We never could produce a white colour from mechanical or chemical action, nor do we consider the experiment with the revolving wheel at all conclusive.

We perceive that light can be arrested, and made to return into itself, or to glance off at certain angles-light, therefore, can be moved. Its particles can be condensed and brought to a focus, by producing among them a rapid rotary motion. If a wheel be turned very rapidly, the rays of light which attached themselves to the spokes of the wheel-and expressed by their density the elevations and depressions of these spokes-do not represent them at all. They close over them, as it were, and become an impervious mass of light, through which the spokes, or the colouring matter, cannot penetrate; the colouring matter being, in fact, all that we can ever see of bodies. If we hold any thing behind the base of the flame of a candle where it swells out filled with gas, we shall see the object distinctly through it; but if we hold the object behind the dense mass of light above the base, the mere density of the light prevents our seeing it. We can see the flame of another candle through the thin stratum of the base, but not in the dense accumulation of the tapering mass.

Light is therefore an obstruction whenever its rays are condensed or accumulated. Light, as well as flame, when accumulated and brought to a focus, does not allow the luminous rays from a body less dense to penetrate its mass. We cannot see any object through flame, or through a dense mass of pure solar light; and it is simply because light and flame are dense and opaque that weaker rays from coloured bodies cannot pass through them. It is only because we cannot see through them that the colours and outlines of bodies, under or behind them, are hidden from us, and not because, as has been imagined, the light which falls on these coloured bodies has decomposed the colouring matter.

If we wink our eyes quickly while looking at a bright window sash, and then shut or cover our eyes, there will be no impression or spectrum of the sash on the cerebral organs of vision. The rays of light which were coming to our eyes from the whole field of view, are by this motion of the eyelids arrested at this point, and their accumulation causes density; this opaque mass closes, as it were, or rather shuts out, the weaker rays that were also on their way to our eyes from the window sash. If we look at the window sash without winking, or shaking the

eyes with the thumb and finger, the whole spectrum of the sash will be seen the moment the eyes are shut.

If it were not for colour and luminousness, a painter could never represent external objects on canvass. Colour gives us the impression of form and outline, and of all the raised and depressed portions of a body. Does the mind, or seeing principle, ever come to the knowledge of real external bodies but through the medium of colour and light? Do we ever really, by vision, perceive any thing but colour and light? When an artist paints the picture of a man or a horse, we see as much as can ever be seen of the originals, excepting their movements. Light, therefore, unless there is a dense accumulation of it between the eye and the coloured body, makes colour visible.

When looking at a picture, we sometimes perceive that the accumulation of light on it is so great that the colours are not seen through it. This density is only perceptible at the angle in which we view the picture; for another person looking at the same picture, at a different angle, will see the colours and form perfectly well, because the light is equably dispersed over it in that direction. The eyes of one person are filled with the dense column of light which falls on the picture, whilst the other person, not in the range of this dense column, sees only the light which falls on it thinly and equably from the whole angle of vision.

The clearing up this mistake of the absolute decomposing property of light, will open the way for deeper researches into the nature and character of light, or luminousness. We shall then acknowledge that light, like all other substances, is opaque when condensed, and transparent when thinly spread out; thus allowing the rays from any illuminated coloured body to pass through with ease. This very quality of opacity is in favour of its materiality.

If colour belonged to light, a greater or less quantity of light, when accumulated at one spot, would not destroy its own powers. If the colour of the substance on which a dense mass of light rested were inherent in light, we should see it in every position of the rays. If coloured rays belonged to the constitution of light, they would return into themselves as simple, luminous rays do, and, like them, be always visible. The colours of the solar spectrum would always be seen in the direction of the luminous rays, following these rays either spirally or as striæ, as is sometimes seen on the "air lines" of the edge of a prism when looking through them at the lamp or fire. Instead of the decomposition of colour by light, it is colour which decomposes light; we now allude to sudden decomposition.

It may be urged that colours fade when long exposed to the

action of light; this is true, remotely. Light and heat combined will cause a change in the colours on the surface of bodies; for instance, a green riband will part with the yellow tint of its combination when exposed to the direct rays of the sun in summer, and yet retain it in winter, although the sun's rays may fall equally strong on it, and light and cold in winter will destroy the blue tint. Even black pigments lose the intensity of their shades when long exposed to light alone. The pigmentum nigrum of the choroids loses its blackness and becomes of a rusty black colour. When the eyes are aged and have been long exposed to the action of the light, this is owing to the friction between the particles of luminousness and dark

ness.

In fact, the more intense the blackness of the colour is, the greater is the commotion between the particles of light and those of colour, and in proportion as the colouring matter approaches blackness, or darkness, will light be decomposed. Colour, therefore, cannot belong to light, for let the material on which the colour rests, or in which it exists, be what it may, whether silk, cotton, wood, iron, or vapour, the destruction of light is the same. It is the black principle itself-blackness, or darkness-that decomposes light, and not the substance in which it is incorporated, or to which it adheres.

That light, or the particles of luminousness, can be moved, that when condensed they become opaque, that they can only pass through transparent substances, are undeniable facts. That light can be moved, and that it can only be transmitted through transparent bodies, has been long known; but as to the fact of density implying obstruction when a rotary motion occurs, that is a doctrine which will not only raise doubts, but strong opposition; we must, therefore, fortify ourselves with all the points that bear upon the subject.

We will first ask what it is that enables us to see external objects, and the answer must be that it is light. Cones of light converge to the eye, and all these cones proceed from elevations and depressions of every object external to the eye. Each spoke of a revolving wheel, and each stripe on a revolving disc, is represented by its own cone of light. If the wheel and disc revolve rapidly, what becomes of these cones, perceiving that light can only move in straight lines? If each ray, or particle of light, were permanently attached to the surface of bodies, it would move round with these bodies, but light can only be bent or moved in one direction, and that is in a straight line.

The wheel revolves, then, rapidly, and as the cones of light cannot move circularly with it, they must have a place somewhere. The question therefore is, where are these cones? they are detached from the colouring matter of the spokes and the

disc, and are not extinguished. Do they still keep their station in front of the revolving wheel-are they divided into a number, or have they all merged into one short cone?

Unquestionably they keep their station and merge into one short cone. From the whole angle of vision the eye receives a large cone of light, throughout which are innumerable smaller cones issuing from external bodies, that appear in this large cone or angle of vision. All these cones, when attached to bodies, converge to the same point on the eye with the large cone, they being a part of the large one, only interrupted in their convergence to the same focus by intervening bodies.

There can be no motion, either quick or slow, through space, without displacing the particles of matter which are traversing or gravitating. But as this space is filled with matter in its latent and perceptible state, if some particles are displaced or driven off, as from the revolving motion of a wheel, others must necessarily rush to the point of friction to supply the place of those that were thus driven off, as an equilibrium is always preserved in space.

When looking at a rapidly revolving wheel, light is closing in upon the surface, and owing to the commotion amongst the particles there is a denser accumulation of it at this spot than there was when the wheel stood still. It is now that light is perceived to be an obstruction, not because it has decomposed the colours of the spokes and stripes, but that, not being attached to the coloured bodies, it converges to the corner without conveying any impression of the objects from whose area it emaThese short cones, which issue from all the elevations and depressions of a surface, when this surface is in a quiet state, can only represent the outline of the circumference when the wheel turns rapidly, and even this outline would be lost to our vision if the whole could revolve with an augmented velocity.

nates.

Light, therefore, is limited in its movements, its action, and its capacity.

It is limited in its movement because it can only proceed in straight lines, the curves of light being mere refractions at short distance, forming short angles.

It is limited in its action, inasmuch as it cannot infringe on black pigments without destroying its own character of luminousness, and it cannot attach itself to bodies that are moving rapidly.

It is limited in its capacity, for it cannot retain its transparent quality when densely accumulated.

If we look at the upper part of the rim of a pair of silver spectacles when the bright light of a window shines on it, the rim presents no obstruction whatever. The object beyond the