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violent, but the return is evident. All means of resisting it, which would have sufficed fifty years since, have become less efficacious.

To sum all in general terms: heredity and selection must produce an alternation of intensity and relief in diseases. That variation must be more marked, when the disease in which it takes place is more fatal, and especially when it attacks youth. Curative or preventive means, which are sufficient in periods of light visitation, lose a portion. of their efficacy at the aggravated periods. And this rule applies particularly to the use of vaccine as a preventive of small-pox.

The works of Darwin being now familiar to physicians, it is probable that many among them have considered the effect of the law of selection upon the variation of intensity in maladies. I doubt, however, whether they have given attention to the consequences relative to vaccination. It is this which has led me to bring within the range of medical investigation an application (perhaps novel) of the ideas of the celebrated English naturalist.

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MODERN OPTICS AND PAINTING.

By O. N. ROOD,

PROFESSOR OF PHYSICS IN COLUMBIA COLLEGE.

II.

ET 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 carefully 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,

FIG. 1.

RED NERVES

GREEN NERVES

RED ORANGE YELLOW GREEN BLUE INDIGO VIOLET

VIOLET NERVES

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

RED

FIG. 2.

YELLOW

GREEN

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 wavelength 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.

FIG. 3.

GREEN

BLUE

VIOLET

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

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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,

FIG. 5.

WHITE

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

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(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

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