This leads us in the next place to ask, "What is light, that agent which is able to produce effects which to a thoughtful mind must always remain wonderful?" A perfectly true answer to this question is, that light is something which comes from the luminous body to us; in the act of vision we are essentially passive, and not engaged in shooting out toward the object long, delicate feelers, as was supposed by the ancients. This something was considered by Sir Isaac Newton to consist of fine atoms, too fine almost to think of, but moving at the rate of 186,000 miles in a second. According to the undulatory theory, however, light consists not of matter shot toward us, but of undulations or waves, which reach our eyes somewhat in the same way as the waves of water beat on a rocky coast.

The atoms, then, which compose a candle flame are themselves in vibration, and, communicating this vibratory movement to other particles with which they are in contact, generate waves, which travel out in all directions, like the circular waves from a stone dropped into quiet water; these waves break finally upon the surface of the retina, and cause in some unexplained way the sensation of sightwe see the candle flame. Substances which are not selfluminous cannot be seen directly or without help; to obtain vision of them it is necessary that a self-luminous body also should be present. The candle flame pours out its flood of tiny waves on the objects in the room; in the act of striking on them some of the waves are destroyed, but others rebound and reach the eye, having suffered certain changes of which we shall speak hereafter.

This rebound of the wave we call reflection; all bodies in the room reflect some of the candle light. Surfaces which are polished alter the direction of the waves of light falling on them, but they do not to any great extent scatter them irregularly, or in all directions. It hence follows that polished surfaces, when they reflect light, present appearances

totally unlike those furnished by surfaces which, though smooth, are yet destitute of polish; the former are apt to reflect very much or very little light, according to their positions, but this is not true to the same extent with unpolished surfaces. The power which different substances have under various circumstances to reflect light is not without interest for us; we shall see hereafter that this is a means often employed by nature in modifying colour.

As a general thing polished metallic surfaces are the best reflectors of light, and may for the most part be considered by the artist as reflecting all the light falling on them. Polished silver actually does reflect ninety-two per cent. of the light falling perpendicularly on it; and though the percentages reflected by steel and other metals are smaller, yet the difference is not ordinarily and easily dis tinguished by an untrained eye.

The case is somewhat different with smooth water: if light falls on it making a small angle with its surface, the amount reflected is as large as that from a metallic surface; while, if the light falls perpendicularly on it, less than four per cent. is reflected. Thus with a clear blue sky and smooth water we find that distant portions of its surface appear very bright, while those at the feet of the observer are of an almost unbelievable dark-blue tint. In this particular instance, the difference between the brightness of near and distant portions of the water is still further exaggerated by the circumstance that the sky overhead is less luminous than that near the horizon; and the distant portions of the sheet of water reflect light which comes from the horizon, the nearer portions that which has its origin overhead. The reflecting power of water is constantly used by artists as a most admirable means of duplicating in a picture a chromatic composition, and easily affords an opportunity, by slight disturbances of its surface, for the introduction of variations on the original chromatic design. It may here be remarked that in actual landscapes con

taining surfaces of still water, it ordinarily happens that the reflected pictures are not exactly identical with those which are seen directly, and the difference may often be considerable. For example, it may easily be the case that an object beyond the water, and situated at some distance from it, is not seen in the reflected picture at all, light from it either not reaching the water, or reaching the water and not being reflected to the eye of the observer.

Polished surfaces, as we have seen, reflect light not only in large quantity, but they as it were press the light well together in rather sharply defined masses; with unpolished surfaces the case is entirely different, the light which falls on them being scattered in all directions. Hence, whereever the eye is placed, it receives some of this light, and a change of position produces far less effect on the quantity received than is the case with light reflected from polished surfaces. Owing to their power of scattering light in all directions, rough surfaces, however situated, never send very intense light to the eye.

If a surface of white linen drapery be illuminated by a dozen different sources, it will reflect to the eye a sample of each kind of light, and what we call its hue will be made up of as many constituents. When we remember that all the different objects in a room reflect some, and usually coloured light, we see that the final tint of our piece of linen drapery depends not only on the circumstance that its natural colour is white, but also on the presence and proximity of curtains, books, chairs, and a great variety of objects; the final colour will hence not be exactly white, but some delicate, indescribable hue, difficult of imitation except by practiced artists. With objects which are naturally coloured, or which show colour when placed in white light, the case becomes still more complicated. Let us suppose that our drapery when placed in pure white light appears red; its hue will

still be modified by the light it receives from objects in the room: for example, if it receives some green light from objects of this colour placed in its neighbourhood, the red hue will incline toward orange; if the added portion of light be yellow, the tendency to orange will be still more marked; on the other hand, light received from blue or violet surfaces will cause the red to pass into crimson or even purple. The grandest illustrations of these changes we find in those cases where objects are illuminated simultaneously by the yellow rays of the sun and the blue light of the clear sky: here, by this cause alone, the natural colours of objects are modified to a wonderful extent, and effects of magical beauty produced, which by their intricacy almost defy analysis. The nature of these changes will be considered in a subsequent chapter, after the principles upon which they depend have been examined.

Finally, it may not be altogether out of place to add that the majority of paintings and chromatic designs are seen by the aid of light which they reflect in a diffused way to the eye of the observer; transparencies, designs in stained or painted glass, etc., are, on the other hand, seen by light which passes entirely through their substance before reaching the eye. Corresponding to this we find that by far the larger proportion of natural objects act upon our visual organs by means of reflected light, while a few only are seen by a mixture of reflected and transmitted light. It hence follows that Nature and the painter actually employ, in the end, exactly the same means in acting on the eye of the beholder. This point, seemingly so trite, is touched upon, as an idea seems to prevail in the minds of many persons that Nature paints always with light, while the artist is limited to pigments: in point of fact, both paint with light, though, as we shall hereafter see, the total amount at the disposal of the painter is quite limited.

In concluding this matter of reflection, we may perhaps be allowed to add that the term reflection is quite frequently confused with shadow-the reflected image of trees on the edge of quiet water being often spoken of as the shadows of trees on the water. The two cases are of course essentially different, a genuine, well-defined shadow on water scarcely occurring except in cases of turbidity.

:

We have seen that all bodies reflect some of the light falling on them; it is equally true that they transmit a certain portion. A plate of very pure glass, or a thin layer of pure water, will transmit all the light falling on it, except that which is reflected; they transmit it unaltered in tint, and we say they are perfectly transparent and colourless substances. Here we have one of the extremes; the other may be found in some of the metals, such as gold or silver it is only when they are reduced to very thin leaves that they transmit any light at all. Gold leaf allows a little light to pass through its substance, and tinges it bluishgreen. Almost all other bodies may be ranged between these two examples; none can be considered absolutely transparent, none perfectly opaque. And this is true not only in a strictly philosophical sense, but also in one that has an especial bearing on our subject. The great mass of objects with which we come in daily contact allow light to penetrate a little way into their substance, and then, turning it back, reflect it outward in all directions. In this sense all bodies have a certain amount of transparency. The light which thus, as it were, just dips into their substance, has by this operation a change impressed on it; it usually comes out more or less coloured. It hence follows that, in most cases, two masses of light reach the eye: one, which has been superficially reflected with unchanged colour; and another, which, being reflected only after penetration, is modified in tint. Many beautiful effects of translucency are due to these and strictly analogous causes; the play