If we wish to project upon the screen L an enlarged image of the little spectrum, we have only to use another suitable lens I in conjunction with K: the diameter of that used by myself is 2-3/4 inches, and its focal length 6-1/2 inches. When we have once found by trial the position in which this supplementary lens gives the clearest image[4] it is easy to arrange a contrivance for removing and replacing it correctly without need of any further adjustment. This apparatus shows then that ordinary white light may be regarded as a mixture of all the variously coloured lights which occur in the spectrum, the sensation produced when it falls upon the eye being consequently a compound one. From these and similar experiments the scientific neophyte is not unlikely to draw an erroneous conclusion. White light, he is apt to think, is _always_ due to the combined action of rays of every possible wave-length, while coloured light consists of rays of one definite wave-length only. Neither of these inferences would be correct. It is not true that white light necessarily contains rays of all possible wave-lengths: the sensation of whiteness may, as will be shown by and bye, be produced quite as effectively by the combination of only two or three different wave-lengths. Nor is it true that such colours as we see in nature are always due to light of a single wave-length; light of this kind is indeed rarely met with outside laboratories and lecture rooms. Far more commonly coloured light consists of mixed rays, and like ordinary white light, it may, and generally does, contain all the colours of the spectrum, but in different proportions. This last assertion is easily proved. By means of a slip of card we may intercept a portion of the little spectrum formed at H (Fig. 3). The dark shadow of the card in the enlarged spectrum on the screen is shown in Fig. 4. It will be noticed that the shadow cuts off a part only of the red, orange, and yellow light, allowing the remainder to pass through the projection lenses. There are still rays of every possible wave-length from extreme red to extreme violet, but the proportion of those towards the red end is less than it was before the card was interposed. [Illustration: _Fig. 4.--Partially intercepted Spectrum._] If now we remove the lens I (Fig. 3) and so mix the colours of this mutilated spectrum, the bright round patch where the mixed rays fall upon the screen will no longer appear white but greenish-blue. If we transfer the card to the other end of the little spectrum, so as to cause a partial eclipse of the violet, blue, and green rays, the colour of the patch will be changed to orange. If we remove the card altogether, the patch will once more become white.