Fig. 11.—Newton’s Disc.
From these experiments it follows that the colours with which all natural substances are clothed, ought not to be looked upon as belonging to them absolutely, but only as a property dependent on the reflection and absorption of light from their surfaces. The leaves of plants, for instance, must not be regarded as being really green in themselves, but as being capable of absorbing certain portions of light, and reflecting others. Grown in the dark, the green substance contained in the plant and its leaves becomes white, and no longer possesses the property of absorbing red light, and reflecting green. A green leaf placed in red light becomes almost black, from its power of absorbing light of that colour; in the blue it reflects a much greater proportion of the coloured ray. A very striking experiment may be performed with a substance known to chemists as the iodide of mercury. If a little of this salt, which is of a brilliant red, be placed in a watch-glass, and heated over a spirit-lamp, it will gradually sublime, and a card held over it will be covered with a number of light yellow crystals. In this case no change of composition has taken place, but simply a change in the power the salt possesses of reflecting some rays and absorbing others. By simply scratching the surface of the card with a pointed piece of wood, the yellow crystals become transformed once more into the red variety; not only this, the transformation gradually spreads, like a red cloud, over the whole of the deposit. There are some other salts known to chemists which possess the property of dichroism, or double colour. The double cyanide of platinum and barium, for instance, appears violet when viewed in one direction, and yellow in another. Change of temperature is often sufficient to change the colour of bodies—white oxide of zinc, for example, becomes bright yellow when heated. Such instances might be supplied ad infinitum, but enough has been said to prove that colour, after all, is only an appearance, and not an essential property of bodies.
We have already spoken of complementary colours, or those which it is necessary to add together in order to produce white light. Blue, for instance, is complementary to orange, red to green, violet to yellow, and vice versa. But it is not by the aid of the palette that this can be proved, for in the case of coloured pigments the arrangement of their atoms interferes in some way with the success of the experiment, and it is only by means of the colours of the spectrum that such recompositions can be effected.
Although most philosophers consider that there are seven colours in the spectrum, there are others who do not admit it, but assert that there are really only three, red, yellow and blue—which by the superposition of their edges produce the intermediate hues of green and orange. Perhaps it would be nearer to the truth to say that the spectrum is composed of an infinite number of colours of different hues.
We have already stated that every one of these colours is indecomposable, and that there are certain worlds illuminated by a single colour only, instead of possessing the infinite number of tints enjoyed by the inhabitants of the solar system. An idea of this effect can easily be gained in a very simple but surprising manner by inserting panes of glass of different colours in the hole of the shutter of a dark room. If the light is yellow, you will find that all those objects that are capable of reflecting yellow light are coloured by it, while those which are bright red or blue become almost black by absorbing the only light present. If we could procure an object which was perfectly complementary in colour to the yellow glass, it would appear perfectly black. The same experiment may be repeated with the other colours. After remaining in this coloured light for some time, if you suddenly pass out into daylight the complementary colour will tinge everything around you.
Instead of using a room into which coloured light only is admitted, lamps burning with a coloured flame may be employed. Brewster mentions the following experiment, which is a very striking one:—Fill a spirit-lamp with alcohol in which has been dissolved as much common salt as the spirit will take up; on being lit it will be found to burn with a livid yellow flame. A room lighted entirely with one or two lamps of this kind will form a laboratory for some very singular experiments. It should, if possible, be hung with pictures in water and oil colours, and the persons present ought to wear nothing but the brightest colours, and the table be ornamented with the gayest of flowers. The room being first lighted with ordinary daylight, the lamps above mentioned should be brought in, and the daylight carefully excluded, when an astonishing metamorphosis will take place. The spectators will be hardly able to recognise each other; the furniture of the room, and every other object contained in it, will reflect but a single colour. The flowers will lose their brilliant tints, the paintings will appear as if they were drawn in Indian ink. The brightest purple, the purest lilac, the richest blue, the liveliest green, will be converted into a monotonous yellow. The same change will take place in the countenances of those present; a livid paleness will spread over their faces, whether young or old, and those who are naturally of an olive complexion will hardly appear changed at all. Every one will laugh at the appearance of his neighbour’s face, without thinking that he is just as great a subject of laughter to them. If, in the midst of the amusement caused by this experiment, the light of day is admitted at one end of the room, the other end being still lighted with the salt-lamp, every one will appear to be half-illuminated with the livid colour which has caused so much surprise, the other portion of their figure and clothes being of the natural hue. One cheek, for instance, will appear animated with its usual brilliancy, while the other will be that of a corpse; one side of a lady’s dress will be brilliant blue or green, as the case may be, the other a colour that it would puzzle an artist to give a name to. The experiment may be varied by admitting the white light through several small holes in the shutter of the room, every luminous spot painting the place where it falls in its natural colours, and the yellow spectators will become spotted with the most singular tints and hues. If a magic lantern is used to throw on the walls of the room and the clothes of the company any luminous figures, such as those of flowers or animals, they will be coloured with these figures in the tint of the wall or fabric upon which they fall, yellowish colours of course escaping the transformation. If nitrate of strontia be substituted for the salt, a crimson tint will be spread over everything. In fact, a lamp prepared in this way will form a source of endless amusement. It is not necessary to use alcohol for the purpose; wood-spirit or methylated alcohol will serve the purpose equally well. If a lamp is not to be had, a few pieces of cotton-wool, tied on wires and dipped in the salted spirit, will do almost as well.
CHAPTER III.
OTHER CAUSES OF COLOUR.
The colours of the spectrum are to the sense of sight what the tones of the gamut are to the sense of hearing. On the one hand, the differences in the lengths of the sonorous waves constitute the variety of note perceptible by the ear; on the other, the differences in the lengths of the luminous waves constitute the variety of colour perceptible by the eye. By and by, we shall learn both the length and rapidity of these vibrations, but it will be as well first to describe the experiments made in this direction by the immortal Newton himself.
Every one has, doubtless, at one period of his life, amused himself with blowing soap-bubbles by means of a tobacco-pipe and a little lather—a sufficiently childish amusement, you will possibly say, but one narrowly connected with the most intricate secrets of the science of optics. These little globes, so fragile that they disappear in a breath, hardly seem worthy of the attention of a thinker, and still less the examination of a philosopher; but it is nevertheless true that Newton made experiments on the colours shown on the surface of these apparently insignificant objects which ended in the most brilliant discoveries, just as on seeing an apple fall he began a train of thought which only terminated in the enunciation of the hypothesis of the earth’s power of gravity.