Popular Science Monthly/Volume 4/March 1874/Modern Optics and Painting II
LET us now pass to the examination of a theory which was proposed in 1807 by the now justly-celebrated Thomas Young who seems to have been gifted with a scientific insight much too keen for the age in which he lived. His views being opposed to the common notions of the day, commanded but little attention, and it was reserved for Helmholtz, almost half a century later, to call attention to this nearly-forgotten theory, and to show that it accounted for all the ascertained facts in a most satisfactory manner. In this work he has been ably seconded by Maxwell, and more lately by the German physicist J. J. Müller, who with improved apparatus care-fully repeated Helmholtz's original experiments, and corrected them in some minor details.
According to our new theory, then, there are in the retina of the eye, where the pictures of external objects fall, three sets of nerves, adapted for the production of three separate, distinct sensations, which we call red, green, and violet. When, owing to any cause whatever, one of these sets of nerves is excited into action, the result is the corresponding sensation; if, for example, we act upon the last set by electricity, pressure, or by the luminous waves, the result will be the sensation of seeing violet light, even though not a ray of light of any kind has actually reached the eye. I think you will admit that the theory is modest in demanding only three sets of nerves, for in the ear, as it seems, there are three thousand nerve-fibrils for the perception of the separate notes. In the eye it would not have been practicable to have employed a separate nerve-fibril for each different tint, for a reason which a moment's thought will render manifest.
But to resume: according to our theory, the first set of nerves responds powerfully to the action of the longer waves, or to that kind of light which we call red; the second set is arranged for waves of medium length, it is strongly set in action by what we call green light; and, finally, the third set is stimulated into action by the shortest waves, or by violet light. Let us for the present call them the red, green, and violet nerves. This diagram shows their relation to the colors of the spectrum (see Fig. 1). As I have just intimated,
these nerves can be set into action by electricity or pressure, and other causes besides light. Taking this into consideration, the next point in the theory will not seem so singular to you: it is, that each set of nerves is capable of being acted on, to a lesser extent, by waves of light not properly belonging to it; so, for example, the set adapted for green light can, to some extent, be stimulated by red light. In a case like this, the sensation will still remain that which we call green, though actually produced by red light. The theory demands this, and the results of experiments on persons who are color-blind to red light are in accordance with it, and presently I hope to give some experimental illustrations of it. The red and violet nerves also have this property, and can be partially set into action by light which does not belong to them, but in each case the sensation remains the one that properly appertains to them.
The last point of the theory is, that, when by any cause all three sets of nerves are excited into action with about the same intensity, the resulting sensation is that which we call white.
We are now in a condition to take up the explanation of the sensations which we call yellow, orange, and blue. Let us suppose for a moment that the eye is acted upon by waves of light shorter than those that produce the sensation of red, but longer than those that give us that of green; referring to Fig. 1, we see that no especial set of nerves has been provided for this case, but a moment's reflection will suggest that these intermediate waves, according to our theory, ought to set into moderate action both the red and green nerves, that the stimulation of the former should predominate as the length of our intermediate waves is made longer; the green set, on the other hand, coming more into play as it is shortened. This accounts, then, for the mode in which waves of a certain length, or light of a certain kind, gives us the sensation of yellow or orange. The light may be simple, and of only one kind, but it produces a compound sensation, made up of the two simple sensations, red and green. From all this it follows that, on the other hand, if we actually present to the same eye mixtures of red and green light, the sensations of yellow or orange ought, according to our theory, to be produced. This is a matter that we can easily test by experiment. With the same apparatus used a moment ago for combining blue and yellow light, I throw upon the screen a large square of red light, and superimpose on it one of green, and, as you see, the result is a fair yellow; on reducing the brightness of the green component, the yellow passes into orange (Fig. 2). I call your attention, in passing, to the circumstance
that, according to the old theory, the result ought not to have been yellow, but rather an approach to white, all the colors, according to its doctrines, being present. Restoring the green squares to their original brightness, and reducing the intensity of the red light, we easily obtain a greenish yellow, completing thus this series of tints.
And now to account for the blue: pure blue light has a wave-length intermediate between that of green and violet light, and hence sets both the green and violet nerves into action, and, though the light itself may be simple, it produces a compound sensation which we call blue. Corresponding to this, I ought to be able to reproduce on the screen blue light by mixing together green and violet light. The experiment is now arranged, and, as you see, we actually do obtain a quite good blue in this way, and can cause it to run through all the changes from greenish blue to violet blue, by altering the intensity of the original components (Fig. 3).
It is easy for us now to understand why, in what I some time ago called our fundamental experiment, yellow and blue light, when mingled, gave not green, but white light; the yellow light stimulated into action the red and green nerves, the blue light the green and violet ones; thus, all three sets of nerves being called into play, the result was of course the sensation of white.
As it will be desirable hereafter to mingle light by the method of revolving disks, it may be well at this point to repeat our fundamental experiment after this fashion, so as to be assured of the correctness of this mode of experimenting. I have placed in front of the lantern a small circular card-board disk, provided with openings over which are fastened pieces of yellow and blue glass (Fig. 4); its
magnified image now covers pretty much the whole screen, and, on causing it to revolve, the colors as you see vanish, and we have in their place a broad circular band of white light (Fig. 5). With a concave mirror, I throw beside it on the screen a direct beam of white light from the lantern, and, if there is any difference, it is in the light from the disk being a little whiter than that of the lantern. The method with revolving disks gives, then, the same result with the more direct one formerly applied, and we can now very conveniently use it for a final test of the new and old theories. Here is a disk cut like the last with open spaces, and armed with red, yellow, and blue glasses. You can predict the result beforehand: it must be white light with added red light—and, as you see, we actually do obtain a broad circular band of red light. Replacing this disk by one provided with glasses capable of transmitting red, green,
and violet light, we find that their mixture actually gives us white light. In all these experiments we have been content with the colored light furnished by stained glasses, but Helmholtz has pushed the investigation much further, and has obtained corresponding results by the use of the pure colored rays of the spectrum.
I called your attention some time ago to the typical mode of expressing the old theory by three intersecting circles of red, yellow, and blue; we have now again on the screen three intersecting circles; the colors are red, green, and violet, with white at the centre
(Fig. 6). It expresses in a condensed form some of the main points of the theory of Young and Helmholtz, and gives us at the same time some of the chief laws of Nature's palette, showing, in a kind of short-hand way, the changes which the tints of surfaces undergo when exposed to a double illumination, or when illuminated by light having a hue different from that of the surface itself. Applications of it will be given at a later stage.
Before leaving this part of the subject, I wish to show a very simple apparatus, with which you can easily repeat for yourselves many of the experiments made tonight, as well as add greatly to their number. It consists merely of a plate of window-glass, of good quality, set up on edge, and fastened on a blackened board (Fig. 1). If the
eye is placed at e, light will come to it directly from the blue square of paper, B, but also at the same time light will reach it from the yellow square of paper, Y; and these two masses of colored light, being mingled on the retina of the eye, will produce the same effects which I have just exhibited to you with much more costly apparatus. You will also find that you can vary the brightness of either of your squares by adjusting them at a greater or less distance from the plate of glass. When they are near to it, the yellow will predominate; the blue, when they are farther from it. Great use was made by Helmholtz of this contrivance in his experiments on this subject, and you will easily be able to prove for yourselves that the red light from paper painted with vermilion, when combined with the green light from the water-colored pigment known as "emerald-green," gives a yellowish or orange tint, according as the apparatus is arranged. Chrome-yellow (the pale variety) and ultramarine-blue give an excellent white. It is somewhat difficult to obtain a good representative of violet from among the colors in use by artists. I find that some samples of the dyeing material known as "Hoffmann's violet BB" answer better than any of the ordinary pigments. If a deep tint of its alcoholic solution be spread over paper, and combined in the instrument with emerald-green, a blue, greenish-blue, or violet-blue, can be readily produced. It is evident that a multitude of experiments of this character can be made, the number of colors united at one time being limited to two. For certain purposes I have modified the apparatus so that three tints can be combined. A second plate of glass is added at P, Fig. 8; this allows the compound beam of light from the first plate to pass, but in addition it reflects to the eye a beam of light from a third slip of colored paper at V; and, by revolving the second glass plate slightly, the intensity of the third beam is easily regulated. This arrangement can be used to produce white light, by the mixture of three colors, for example, vermilion, emerald-green, and the violet just mentioned.
Let us pass, in the next place, to the consideration of another class of facts, which have an important bearing on our subject. If you illuminate some such object as a sheet of paper with a very moderate light, then, upon doubling the amount of light falling on it, it is possible that the paper, in the second case, may appear to you twice as bright as it did at first. But, if this process be for some time continued, you will soon come to a point where doubling the actual illumination produces very little effect, and finally a stage will be reached where a very great increase of actual illumination produces no additional effect on the eye at all, your paper looking no brighter than in a much feebler light. Let me make an experiment, to at least partially illustrate this: We have now upon the screen four large squares of white light, and they are, as you see, all of equal brightness. But, by turning this Iceland-spar prism, I superimpose one of the squares upon its neighbor; the central square now seems rather brighter than its companions, but I think no one in this room would suspect that its actual illumination was twice as great as that of the others. To take a still more striking example out of your own experience: you have often noticed the reflection of the gas-flames in the streets against the four panes of glass used to protect them, and have seen that the real flame looks brighter than the reflected one; but who would suppose that its actual luminosity was more than eleven times greater than that of its companion? In point of fact, sensation does not, for the most part, increase as rapidly as the actual intensity of the light exciting it, and a point can finally be reached where sensation does not increase at all, even though the actual brightness of the light is greatly multiplied. Our nervous organization is, in this direction, limited and finite, just as it is in all others.
The next matter to which your attention is called is really allied to the preceding, though, at first sight, the connection is not very evident. Any color, if very luminous, seems paler than it really is. This simple piece of apparatus, where a bat-wing gas-flame is placed between a sheet of card-board and a plate of stained glass, will serve for experimental demonstration. The glass is red, and the paper seen through it appears of a deep-red hue, but the gas-flame itself, being much more luminous than the paper, does not look red at all; its tint is orange (Fig. 9). Replacing the red glass by green,
we have the paper appearing with a deep-green hue, while the flame seems greenish-yellow. Let us see, if we can explain these curious changes of tint by Young's theory. The red glass used in the first experiment transmits to the eye only red light, or light capable of stimulating mainly the red nerves; but, if we increase its intensity beyond a certain point, its action on the red nerves begins to flag, and we soon have a state of things where a further increase of the red light produces no effect at all on the red nerves, they being already stimulated up to the maximum point. But, according to our theory, this red light has all along been acting, to some extent, on the green, and to a less extent on the violet nerves; and, as we add to its intensity, it acts still more powerfully on them, so that especially the green nerves come more and more into play, and a green is added to the original red sensations; the result, of course, is the sensation of orange.
The explanation of the tint obtained in the other experiment is quite similar. The green nerves are first stimulated up to their maximum point by green light of a certain strength, a further increase of its intensity brings into play the other two sets of nerves, particularly the red, and the tint quite naturally becomes greenish-yellow. You remember that, in a previous experiment, we found that a mixture of much green with a little red light gave a greenish-yellow. The nerves for violet light always lag behind the others, as will afterward be shown by a particular experiment.
The general effect, then, of a very bright illumination on natural objects is to cause their colors to appear paler than they otherwise would. This is, indeed, to the painter, a precious resource, for representing, in his pictures, high degrees of luminosity, and is often employed with most happy effect. According to the careful experiments by Aubert, white paper is only fifty-seven times lighter than black paper, and the painter is in the predicament of being obliged to represent the vast range of natural illumination within these very narrow limits; hence the desirability of employing an artifice of this kind to overcome a difficulty which, if fairly met, would prove insuperable.
The considerations that I have just presented explain to us, quite readily, the curious circumstance that light of any color, if very bright, is at last accepted by the eye for white, all three sets of nerves finally reaching, in the order indicated, their point of maximum stimulation. You can repeat for yourselves a simple experiment of Helmholtz's, in this connection: hold before the eyes, for some little time, a plate of stained glass; the color may be red, yellow, blue, or green; after a while you will come to consider the brightest objects in your field of view white; as, for example, a gas-flame, the sky, or white paper. In point of fact, to be quite frank, white is only a relative sensation, and, if any thing like equality of stimulation is produced in the three sets of nerves, we finally accept the tint for white. I have especially arranged an experiment to illustrate this point: We have now upon the screen two large squares of light;' one is deep red, the other green: I remove from the lantern a large plate of green glass; the red square has retained its color, and is now brighter, but the other square has become white or almost white. On removing the red glass, the red square on the screen is replaced by a white one, and we now for the first time see that its companion, which a moment ago we were ready to take for white, has a decidedly green hue; in fact, all the while the light producing it has been passing through a plate of pale-green glass, which was behind the others. Let me take away this plate, and now at last we have both our squares illuminated with pure white light. Is this light really white? Not at all; it has been tinged decidedly yellow, by passing through a pale-yellow glass, which has been concealed in the apparatus all the time as a reserve, and, on removing this glass, we find that the light we were ready to accept for white looks yellow, when compared with the purer light of the lantern. Finally, if we could throw a sample of daylight on the screen, we should again see that the light of the lantern itself is not white, but yellowish. White is evidently only a relative sensation.
In some of the preceding experiments it has been seen that, as we increase the actual brightness of any colored light, red for example, so does the sensation produced also increase, but usually at a slower rate. Now, it happens that some of the sensations increase more rapidly than others; for example, the sensation for red or yellow increases more rapidly than that for blue or violet. In fact, as I said some time ago, the violet nerves always lag behind. From this it happens that, if we place side by side a quantity of blue and red light, arranging matters meanwhile so that they appear to the eye to be of equal brightness, then, upon adding considerably but equally to their actual luminosity, it will turn out that the red light will quite outstrip in apparent brilliancy its rival. We have now two such squares of red and blue, side by side on the screen, and it is difficult to say which is the brighter; but, when I greatly increase their illumination, it becomes evident that the blue one has been beaten; or, better still, when I reverse the experiment, starting with red and blue squares, of equal and considerable brilliancy, then, upon turning down the light of the lan-tern, and rendering them both dark, the blue square remains visible after its red companion has vanished. As another example, I may mention the blue color of the sky, which still continues plainly perceptible at night, when the illumination is so feeble that other colors have disappeared. Dove has pointed out that, in picture-galleries, as the light of day fades out, the blue colors in draperies and skies retain their power longer than the reds and yellows.
It is owing to this circumstance that, in actual landscapes, seen under the comparatively feeble light of the moon, there is a prevailing tendency to blueness. This also explains the circumstance that a landscape, illuminated by bright white clouds, appears more yellow in general hue than when the clouds are not bright, though still retain-in their whiteness, the strong white light stimulating more powerfully the sets of nerves concerned in the production of yellow. I think we all know that, on dark, dull days, there seems to be a tendency to blueness in the coloring, even though we may not have paid much attention to the reverse phenomenon. All this is prettily illustrated by a very simple experiment of Helmholtz's, who noticed that the impression of a bright day was produced by merely holding a pale-yellow glass before his eyes, the tint of the glass being so faint as hardly to disturb the natural colors of the objects; the use of a very pale-blue glass seemed, on the other hand, to darken up the landscape, as though a cloud were passing over the scene.