Another experiment in the same direction consists in reuniting the colours by causing them to pass through a double convex lens, behind which is placed a screen of ground glass, or a card ([fig. 8]). By advancing and withdrawing this screen we can easily find the exact spot where the rays reunite, and form a dazzling spot of white light. This point is called the focus, from a Latin word, signifying “fire-place,” a term which will put the student in mind of the frequently repeated experiment of burning a piece of paper with an ordinary magnifying-glass.
Instead of using a lens, you can, if you please, employ a concave mirror, using the ground glass or cardboard screen, as before. The colours reflected by the mirror unite at its focus, and produce a brilliant white spot in just as conclusive a manner as in the other experiment.
Fig. 9.—Recomposition of Light by means of a Concave Mirror.
A fourth experiment, which is somewhat more difficult for the student to accomplish, consists in causing every one of the seven different colours to be reflected from a separate mirror.
The mirrors in this case are concave, and are so mounted as to be capable of being moved in any direction. By directing each of the seven rays, one by one, upon the same point, you may observe the gradual decomposition of the coloured light. The effect obtained by adding the last colour to the mixture is quite magical, the white circle being produced from two brilliantly-coloured spots.
Fig. 10.—Recomposition of Light by means of a number of Mirrors.
A fifth experiment, first devised by Newton, is also within the reach of the student. On a disc of cardboard the centre and border of which have been previously painted black, are pasted seven strips of paper, painted as nearly as possible of the same colour as the components of the spectrum—or if the student is anything of an artist he may paint the disc in imitation of the spectrum, carefully shading off the tints into each other. If the disc be now rapidly rotated the colours will disappear, and a greyish hue will be seen, which will approach more closely to white, the nearer the colours on the disc are to those of the spectrum. This experiment is not precisely the same in principle as the preceding ones, for it is evident that the colours on the disc do not mix, but only the impressions they form upon the retina. We have already said that such impressions remain on the eye for one-tenth of a second or there-abouts; the disc must therefore revolve at least ten times a second, or the effect will not be perceived.
