CHAPTER VI.
the rainbow—decomposition of white light by the prism—formation of primary and secondary bows—rainbows in mountain regions—the rainbow a sacred emblem—lunar rainbow—light decomposed by clouds—their beautiful colours—examples.
By means of rain and rain clouds we get that beautiful appearance so well known as the rainbow. In order to form some idea of the manner in which the rainbow is produced, it is necessary to know something of the manner in which light is composed. Sir Isaac Newton was the first philosopher who clearly explained the composition of light, as derived from the sun. He admitted a ray of the sun into a darkened room through a small hole in the window shutters; in front of this hole he placed a glass prism, and at a considerable distance behind the prism he placed a white screen. If there had been no prism between the hole and the screen, the ray of light would have proceeded in the direction of the dotted lines, and
a bright spot would have fallen upon the floor of the room, as shown in the figure. But the effect of the prism is to refract or bend the ray out of its ordinary course, and in doing so it does not produce a white spot upon the screen, but a long streak of beautiful colours, in the order marked in the figure, red being at the bottom, then orange, yellow, green, blue, indigo, and violet at the top.
In order to account for the production of these colours from a ray of light, Newton supposed that such a ray is actually made up of seven distinct colours, which being mixed in proper proportions neutralize or destroy each other. In order to account for the decomposition of the ray of white light by the prism, and for the
lengthened form of the spectrum, as it is called, he supposed that each of the seven coloured rays was capable of being bent by the prism in a different manner from the rest. Thus, in the figure, the red appears to be less bent out of the direction of the original ray than the orange—the orange less than the yellow, and so on until we arrive at the violet, which is bent most of all.
It is scarcely necessary to remark, that these views were found to be correct, except as regards the number of colours in the solar spectrum; for it is now ascertained, with tolerable certainty, that there are only three primitive or pure colours in nature, and these are red, yellow, and blue; and it is supposed that by mingling two or more of these colours in various proportions, all the colours in nature are produced.