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.
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.
It follows a fortiori that when any portion of the little spectrum is eclipsed totally, instead of only partially, the light from the remainder will appear, when combined, to be coloured. Very beautiful changes of hue are exhibited by the bright patch when a narrow opaque strip, such as the small blade of a pocket knife, is slowly moved along the little spectrum at H, eclipsing different portions of it in succession. The patch first becomes green, then by imperceptible gradations it changes successively to blue, purple, scarlet, orange, yellow, and finally, when the knife has completed its course, all colour disappears and the patch is again white.
We may improve upon this crude experiment, and, after Captain Abney’s plan, prepare a number of small cardboard stencils, with openings corresponding to any selected parts of the little spectrum. When a card so prepared is placed at H ([Fig. 3]) the bright patch upon the screen is formed by the combination of the selected rays, all the others being quenched. We shall find that under these conditions the bright patch is generally, but not always, coloured.
