Before embarking on an explanation of these three methods it will be necessary to examine briefly the nature of those phenomena on which all are based--light and colour. The two are really identical, light is colour and colour is light. Scientists now agree that the sensation of light arises from the wave-like movements of that mysterious fluid, the omnipresent ether. In a beam of white light several rates of wave vibrations exist side by side. Pass the beam through a prism and the various rapidities are sorted out into violet, indigo, blue, green, yellow, orange and red, which are called the pure colours, since if any of them be passed again through a prism the result is still that colour. Crimson, brown, &c., the composite colours, would, if subjected to the prism, at once split up into their component pure colours. There are several points to be noticed about the relationship of the seven pure colours. In the first place, though they are all allies in the task of making white light, there is hostility among them, each being jealous of the others, and only waiting a chance to show it. Thus, suppose that we have on a strip of paper squares of the seven colours, and look at the strip through a piece of red glass we see only one square--the red--in its natural colour, since that square is in harmony only with red rays. (Compare the sympathy of a piano with a note struck on another instrument; if C is struck, say on a violin, the piano strings producing the corresponding note will sound, but the other strings will be silent.) The orange square suggests orange, but the green and blue and violet appear black. Red glass has arrested their ether vibrations and said “no way here.” Green and violet would serve just the same trick on red or on each other. It is from this readiness to absorb or stop dissimilar rays that we have the different colours in a landscape flooded by a common white sunlight. The trees and grass absorb all but the green rays, which they reflect. The dandelions and buttercups capture and hold fast all but the yellow rays. The poppies in the corn send us back red only, and the cornflowers only blue; but the daisy is more generous and gives up all the seven. Colour therefore is not a thing that can be touched, any more than sound, but merely the capacity to affect the retina of the eye with a certain number of ether vibrations per second, and it makes no difference whether light is reflected from a substance or refracted through a substance; a red brick and a piece of red glass have similar effects on the eye. This then is the first thing to be clearly grasped, that whenever a colour has a chance to make prisoners of other colours it will do so.