CHAPTER II.
THE SOLAR SPECTRUM.
The white light that the glorious orb of day spreads over the face of nature is the original source of all those brilliant and sombre colours with which the works of the Creator are beautified. To the rays of the sun we owe not only the whiteness of the lily, but the scarlet of the field poppy, the modest blue of the timid violet, the splendour of the peacock’s plumage, the cool green of the meadows, and the purple and gold of the distant mountains. For, as we have hinted before, this white light, which seems of itself so destitute of colour, is productive of every hue that the eye of man is capable of appreciating.
It may seem that I am bestowing too much praise upon our own sun; but if you are surprised that I should seek to exalt this brilliant globe of ever-burning fire, I must ask you to recollect, that though the starry heavens are full of suns as vast and important as ours, and possibly affording brilliant colourless light to worlds full of inhabitants, there are others that give forth rays that are far from being white. Some are as green as emeralds, others are as blue as sapphires, while others give out a warm light like a ruby or topaz. The worlds which surround these can only receive light of a certain colour, or at any rate they are restricted to a few shades and hues. Imagine living in a world where everything was always couleur de rose, or in which the inhabitants were continually looking blue! A residence in either of them for a short time would undoubtedly cause us to appreciate the relative value of our own little sun, small as it is in comparison with some of the mighty orbs floating about in space.
The fact that the light of the sun is the source of all the changing hues to be found on the surface of the earth season after season was first discovered by Newton, and his experiments are easily repeated with a very few and inexpensive appliances.
A small round hole is made in the window-shutter of a room, facing the sun, and the pencil of light proceeding from it is allowed to fall upon the surface of a three-sided prism, held in a horizontal position, and placed at a distance of a few inches from the aperture ([fig. 5], Frontispiece). The pencil of light does not pass through the prism as if it were a plate of glass with parallel sides, but in virtue of the laws of refraction, of which we have already spoken, it is turned out of its natural course, and is thrown upon the wall in the direction indicated in the figure. The pencil of light is not only turned aside, but it is also widened out into a band which is truly painted with all the colours of the rainbow, every tone and hue being of the most marvellous brilliancy. This long coloured stripe, which constitutes one of the most beautiful sights that the science of optics can afford us, is known to scientific men by the name of the solar spectrum.
Before going into the causes that produce these colours, let us first examine their number and position. Beginning at the top, we shall find that they run in the following order:—Violet, indigo, blue, green, yellow, orange, red. The red being lowest is called the least refrangible of them all; or, in other words, in passing through the prism it was bent less out of its course than its companions. Violet, being at the top, is of course the most refrangible. The cause of the separation of the colours of white light is consequently only the effect of their individual character. They were, so to speak, so many streams flowing together until an unexpected deviation in their course caused them to separate. This change in the direction of their flow brought out their personal individuality, and they at once became completely disunited.
Every single tint in the prismatic spectrum is simple, and cannot be decomposed. This may be shown by passing any of them through another prism, when it will be found that no change will take place in the colour or size of the pencil. Hence those worlds already spoken of, whose light of day is red, blue, or green, never see any colours but these. ([Fig. 6], Frontispiece).
It is just as easy to reunite the colours into which white light is decomposed, by applying a second prism in a reversed position to the pencil of coloured light, as it is to separate them in the first instance. The method of accomplishing this is shown in [fig. 7], Frontispiece.
Fig. 8.—The Recomposition of Light.