come out of the drop, the red rays are quite separated from the blue, and when the beam reaches its destination, the various colours enter the eye separately, forming a line of variously coloured light, the upper part red and the lower part blue, instead of a mere point of white light, as the ray would have appeared if seen before it entered the drop. The eye naturally refers each part of the ray to the place from whence it appears to come, and thus, with a number of drops falling and the sun not obscured, a rainbow is seen, which represents part of a number of concentric circular lines of colour, the outermost of which is red, the innermost violet, and the intermediate ones we respectively name orange, yellow, green, blue, and indigo.
It has also been found by careful experiment, that these are not all pure colours, most of them being mixtures of some few that are really primitive and pure, and necessarily belong to solar light. It is these mixed in due proportion which make up ordinary white light, which is the only kind seen when the sun's rays have not undergone this sort of decomposition or separation into elements. The actual primitive colours are generally supposed to be red, yellow, and blue, and much theoretical as well as practical discussion has arisen as to how these require to be mixed, what proportion they bear to each other in their power of impressing the human eye, and many other matters for which we must refer to Mr. Field, Mr. Owen Jones, and others, who have studied the subject and applied it.
In a general way it is found convenient to remember, or rather to assume, that three parts of red, five parts of yellow, and eight parts of blue form together white, and, therefore, that the pencil of white light contains three rays of red, five of yellow, and eight of blue. To produce the other prismatic colours, we must mix red with a little yellow to form orange; yellow with some blue to form green; much blue with a little red to form indigo, and a little blue with some red to form violet. In performing experiments on colour it is convenient, instead of a drop of water, to substitute a prism of glass in decomposing the rays of light. We may thus produce at will a convenient image, called a prismatic spectrum, which, when thrown on a wall, is a broad band of coloured lights, having all the tints of the rainbow in the same order. Looking at this image, the red is at the top and the violet at the bottom, and it may be asked, How does the red get amongst the blue to form violet, if the red rays are bent up to the top of the spectrum? The answer is, that a quantity of white light not decomposed, and a part of all the colour rays, reach all parts of the spectrum, however carefully it is sheltered, but that so many more red rays get to the top, so many more of the yellow to the middle, and so many more blue to where that colour appears most brilliant, that these are seen nearly pure, whilst where the red and yellow or yellow and blue mix they produce distinct kinds of colour, and where the blue at the bottom is faint, and some of those red rays fall that do not reach the red part of the spectrum, the violet is produced. In point of fact, therefore, all the colours of the spectrum, as seen, are mixtures of pure colour with white light, while all but red are mixtures of other pure colours with some red and some yellow
