green field surrounding it, and hence was accepted for this tint. The same pale blue-green slip, when placed on a palereddish ground, assumed a stronger blue-green hue than when on a white ground. In the first of these experiments we have an illustration of harmful and in the second of helpful simultaneous contrast. The result in both cases coincided with that which successive contrast would have produced under similar circumstances.

It has been stated above that the effects produced by simultaneous contrast are due not to retinal fatigue, but to deception of the judgment; now, as the effects of simultaneous contrast are identical in kind with those generated by

successive contrast, it is evident that the statement needs some proof. This can be furnished with the aid of a beautiful experiment with coloured shadows. In making this experiment we allow white daylight to enter a darkened room through an aperture, A, arranged in a window, as indicated in Fig. 121. At R we set up a rod, and allow its shadow to fall on a sheet of white cardboard, or on the

white wall of the room. It is evident now that the whole of the cardboard will be illuminated with white light, except those portions occupied by the shadow 1. We then light the candle at C, Fig. 122; its light will also fall on

the cardboard screen, and will then cast the shadow 2; that is, the candle-light will illuminate all parts of the screen except those occupied by the shadow 2; this portion will be illuminated with pure white light. Instead, however, of appearing to the eye white, the shadow 2 will seem to be coloured decidedly blue. For the production of the most powerful effect, it is desirable that the shadows should have the same depth, which can be effected by regulating the size of the aperture admitting daylight. Now, although the shadow cast by the candle is actually pure white, yet, by contrast with the surrounding orange-yellow ground, it is made to appear decidedly blue. So strong is the illusion that, even after the causes which gave rise to it have disappeared, it still persists, as can be shown by the following experiment of Helmholtz: While the coloured shadows are

falling on the screen, they are to be viewed through a blackened tube of cardboard, held in such a way that the observer has both the shadows in his field of view; the appearance then will be like that represented in Fig. 123.

After the blue shadow has developed itself in full intensity, the tube is to be moved to the left, so that the blue shadow may fill the whole field. The tube being held steadily in the new position, the shadow will still continue to appear blue instead of white, even although the exciting cause, viz., the orange-yellow candle-light, is no longer acting on the eye. The candle may be blown out, but the surface will still appear blue as long as the eye is at the tube. On removing the tube, the illusion instantly vanishes, and it is perceived that the colour of the surface is identical with that of the rest of the screen, which is at once recognized as white. In a case like this the fatigue of the retinal elements can play no part, as the illusion persists during a far longer period of time than is necessary for their complete rest; we must hence attribute the result to a deception of the judgment. Expressing this in the language of Young's theory, we say that the sensation of white is produced when the three sets of nerves, red, green, and violet, are

stimulated to about the same extent; but that nevertheless, as we have in ourselves no means of judging with certainty about this equality of stimulation, we may under certain circumstances be induced to accept an unequal for an equal stimulation, or the reverse. In the experiment with the coloured shadows we had before us in the shadow due to the candle-flame an equal stimulation, which by contrast we were in the first instance induced to accept as unequal, and the judgment afterward obstinately persisted in the error till it was corrected and took a new departure.

This experiment may be modified and extended by the use of coloured glasses instead of a candle-flame. The window is to be provided with two apertures, one of which is to be covered with a piece of stained glass, through which sunshine will be admitted to the darkened room; the other aperture will admit white light, as before. If red glass be employed, the colour of the shadows will appear red and greenish-blue. In each case the shadows will assume complementary colours.

The effects of simultaneous contrast can also be studied with the aid of a contrivance of Ragona Scina. Two sheets of white cardboard are attached to a couple of boards fastened together at a right angle, as indicated in Fig. 124. Between the boards a plate of rather deeply coloured glass, G, is to be held in the manner shown in the figure, so that it makes, with the vertical and horizontal cardboards, an angle of about 45°. If the eye is placed at E, two masses of light will be sent to it. From the vertical cardboard white light will start, and, after being reflected on the glass plate G, will reach the eye. This light will be white, or almost entirely white, even after suffering reflection, owing to the circumstance that, with a deeply coloured plate of glass, the reflection takes place almost entirely from the upper surface, or from that turned toward the light. The second mass of light will proceed from the horizontal plate H: originally of course it was white light, but on its way

to the eye it traverses the glass plate, and becomes coloured by absorption. If the glass plate is red, this light when it reaches the eye will of course have the same colour; consequently the first result is that we have presented to the eye

B

G

FIG. 124.-Apparatus of Ragona Scina for Contrast.

a mixture of red with white light, which will give the observer the idea that he is looking at an horizontal, square field of a somewhat pale reddish tint. If now a small black square be attached to the vertical cardboard at B, of course no white light can come to the eye from this portion of the cardboard, and the image of this spot will seem to the eye to be at b, on the horizontal board under the eye. Under ordinary circumstances this image would appear black; in point of fact, however, in this case it appears deep red, owing to the red light transmitted by the plate of glass. Thus far the arrangement amounts to a device for presenting to the eye a mixture of red with white light, the white light being absent at a certain spot, which consequently appears of a deeper red. A similar black square is now to be placed on the horizontal board at c; it will of