The six most prominent hues of the spectrum are, in fact, supplemented by an immense multitude of subordinate ones, the total number which the eye can recognise as distinct being not less than a thousand. All the colours that we see in nature, with the exception of the purples (about which I shall say more presently), are here represented, and every single variety of tone in the prismatic scale corresponds with one, and only one, definite wave-length of light. The source of all these colours is, as we know, a beam of white or colourless light, the constituents of which have been sorted out and arranged so that they fall side by side upon the screen in the order of their several wave-lengths. If, then, these coloured constituents were all mixed together again, it would be reasonable to expect that pure white light would be reproduced. The experiment has been performed in a great many different ways, several of which were devised by Newton himself, and the result admits of no doubt whatever. The method which I intend to describe is not quite so simple as some others, but it has great advantages in the way of convenient manipulation, and affords the means of demonstrating a number of interesting colour effects in an easily intelligible manner. By the simple operation of moving aside a lens out of the track of the light, we can gather up and thoroughly mix together all the variously coloured rays of the spectrum and cause them to form upon the screen a bright circular patch, which, though due to a mixture of a thousand different hues, is absolutely white. When the lens is replaced, which is done in an instant, the mixture is again analysed into its component parts, and the spectrum reappears. The arrangement of the apparatus, which is essentially the same as that devised by Captain Abney, and called by him the "colour-patch apparatus," is shown in the annexed diagram (Fig. 3). [Illustration: _Fig. 3.--Abney's Colour-patch Apparatus._] The light of an electric lamp A placed inside the lantern is concentrated by the condensing lenses B upon a narrow adjustable slit C. The framework of this slit is attached to one end of a telescope tube, which carries at the other end an achromatic lens D of about 10 inches focus. The rays having been rendered parallel by D are refracted by the prism E; they then pass through a circular opening in the brass plate F to the lens G, the focal length of which is 7 inches, and form a little bright spectrum upon a white card held in a grooved support at H. The card being removed, we place at K a lens having a diameter of 5-1/2 inches and a focal length of 18 inches or more, and adjust it so that a sharply defined image of the hole in the brass plate F is formed upon the distant white screen L. If all the lenses are correctly placed, this image, though formed entirely by the rays which constituted the little spectrum at H, will be perfectly free from colour even around the edge.