Curiosities of Light and Sight

by Shelford Bidwell

Fig. 5.—Stencil Cards.

The first diagram in [Fig. 5] represents a blackened card, which allows only the red and a little of the orange to pass through. When this is inserted in the grooved holder at H, the bright patch immediately turns red. The second diagram shows another, which transmits the middle portion of the spectrum, but blocks the red and the violet at its two ends: with this card the colour of the patch becomes green. The third card has openings for the violet and the red rays: this turns the patch a beautiful purple, a hue which, as already mentioned, is not produced by light of any single wave-length. The purples are mixtures of red and violet or of red and blue.

Now I have in my possession three pieces of glass (or, to be strictly accurate, two pieces of glass and one glass-mounted gelatine film) which, when placed transversely in the beam of light, either at H ([Fig. 3]) or anywhere else, behave exactly like these three cardboard stencils. The first glass cuts off all the spectrum except the red and part of the orange, just as the first stencil does, though the line of demarcation is not quite so sharp. This is in fact a piece of red glass, or in other words the light that it transmits produces the sensation of red. The second glass, like the second stencil, allows the whole of the spectral rays to pass freely except the red and the violet, which disappear as if they were obstructed by an opaque body. This is a green glass. And the third (which is really a film of gelatine) cuts out the middle of the spectrum but transmits the red and violet ends. The colour of the gelatine is purple.[5]

The glasses and the gelatine in question act like the cardboard stencils in completely cutting off some of the spectral rays and transmitting others, and they owe their apparent colours to the combined influence which the transmitted rays exert upon the eye. Many other coloured glasses merely weaken some of the rays, without entirely quenching any. A piece of pale yellow glass, for example, when placed in the path of the beam of light from which the spectrum on the screen is formed, simply diminishes the brightness of the blue region and does not wholly quench any of the rays; and again, a common kind of violet-coloured glass enfeebles, but does not quite obliterate, the middle portion of the spectrum.

From such observations as these we infer that the glasses derive their respective colours from the light which falls upon them. The first glass would not appear red if seen in a light which contained no red rays. This is easily proved by an experiment with the colour-patch apparatus. The spectrum being once more combined into a bright white patch (which turns red if the glass is for a moment interposed), let all the red rays and part of the orange be cut off with a suitable stencil. The re-combined light is no longer white but greenish-blue, as is evidenced by the colour of the patch; and nothing that is illuminated by this light can possibly appear red. The piece of red glass, if placed in the beam, will now cast a perfectly black shadow, and a square of bright red paper held in the middle of the patch will look as black as ink. It will be shown later how we may obtain light which, although it appears to the eye to differ in no respect from ordinary white daylight, yet contains no red component, and is consequently as powerless as this greenish-blue light to reveal any red colour in the objects which it illuminates.

If we substitute a stencil which admits only red rays, we shall obtain a beam of light in which no colour but red can be seen. Green and blue glasses when exposed to this light will cast black shadows, while pieces of green and blue paper will become either black or dark grey.

We see then that the colours of transparent objects, like the glasses used in these experiments, are brought out by a process of filtration. Certain of the coloured ingredients of white light are filtered out and quenched inside the glass, and it is to the remaining ingredients which pass through unimpeded that the observed colour is due. The energy of the absorbed rays is not lost of course, for energy, like matter, is indestructible. It is transformed into heat. A coloured glass held in a strong beam of light will in a short time become sensibly warmer than one that is clear and colourless.

In studying colour effects as produced by coloured glasses, we have at the same time been learning how the great majority of natural objects—not only those which are transparent but also those called opaque—become possessed of their colours. For the truth is that few things are perfectly opaque. When white light falls upon a coloured body, it generally penetrates to a small depth below the surface, and in so doing loses by absorption some of its coloured components, just as it does in passing through the pieces of glass. But before it has gone very far—generally much less than a thousandth part of an inch—it has encountered a number of little reflecting surfaces due to optical irregularities, which turn the light back again and compel it to pass a second time through the same thickness of the substance: it thus becomes still more effectively sifted, and on emerging is imbued with a colour due to such of the components as have not been quenched in the course of their double journey through a superficial layer of the substance.

Any coloured rays reflected by an object must necessarily be contained in the light by which the object is seen. The following is a curious experiment illustrating this.

A large bright spectrum is projected upon a screen and in the green or blue portion of it is held a wall poster. The letters and figures upon the paper are seen to stand out boldly as if printed with the blackest ink. But if the poster is moved into the red part of the spectrum, the printing at once disappears as if by magic, and the paper appears perfectly blank. The explanation is that the letters are printed in red ink—they can reflect no light but red. Green or blue light falling upon them is absorbed and quenched, and the letters consequently appear black. On the other hand when the poster is illuminated by the red rays of the spectrum, the letters reflect just as much light as the paper itself, and are therefore indistinguishable from it.