第63章

In short, the apparent change in color or brightness through contrast is due to no change in excitation of the organ, to no change in sensation; but in consequence of a false judgment the unchanged sensation is wrongly interpreted, and thus leads to a changed perception of the brightness or color.

In opposition to this theory has been developed on which attempts to explain all cases of contrast as depend- ing purely on physiological action of the terminal apparatus of vision.

Hearing is the most prominent supporter of this view.By great originality in devising experiments and by insisting on rigid care in conducting them, he has been able to detect the faults in the psychological theory and to practically establish the validity of his own.Every visual sensation, he maintains, is correlated to a physical process in the nervous apparatus.

Contrast is occasioned, not by a false idea resulting from unconscious conclusions, but by the fact that the excitation of any portion of the retina -- and the consequent sensation depends -- not only on its own illumination, but on that of the rest of the retina as well.

"If this psycho-physical process is aroused, as usually happens, by light-rays impinging on the retina, its nature depends not only on the nature of these rays, but also on the constitution of the entire nervous apparatus which is connected with the organ of vision, and on the state in which it finds itself."

When a limited portion of the retina is aroused by external stimuli, the rest of the retina, and especially the immediately contiguous parts, tends to react also, and in such a way as to produce therefrom the sensation of the opposite degree of brightness and the complementary color to that of the directly-excited portion.When a gray spot is seen alone, and again when it appears colored through contrast, the objective light from the spot is in both cases the same.Helmholtz maintains that the neural process and the corresponding sensation also remain unchanged, but are differently interpreted ; Hering, that the neural process and the sensation are themselves changed, and that the 'interpretation' is the direct conscious correlate of the altered retinal conditions.According to the one, the contrast is psychological in its origin; according to the other, it is purely physiological.In the cases cited above where the contrast-color is no longer apparent -- on a ground with many distinguishable features, on a field whose borders are traced with black lines, etc., -- the psychological theory, as we have seen, attributes this to the fact that under these circumstances we judge the smaller patch of color to be an independent object on the surface, and are no longer deceived in judging it to be something over which the color of the ground is drawn.The physiological theory, on the other hand, maintains that the contrast-effect is still produced, but that the conditions are such that the slight changes in color and brightness which it occasions become imperceptible.

The two theories, stated thus broadly, may seem equally plausible.Hering, however, has conclusively proved, by experiments with after-images, that the process on one part of the retina does modify that on neighboring portions, under conditions where deception of judgment is impossible. A careful examination of the facts of contrast will show that its phenomena must be due to this cause.In all the cases which one may investigate it will be seen that the upholders of the psychological theory have failed to conduct their experiments with sufficient care.

They have not excluded successive contrast, have overlooked the changes due to fixation, and have failed to properly account for the various modifying influences which have been mentioned above.We can easily establish this if we examine the most striking experiments in simultaneous contrast.

Of these one of the best known and most easily arranged is that known as Meyer's experiment.A scrap of gray paper placed on a colored background, and both are covered a sheet of transparent white paper.The gray spot then assumes a contrast-color, complementary to that of the background, which shines with a whitish tinge through the paper which covers it.Helmholtz explains the phenomena thus:

"If the background is green, the covering-paper itself appears to be a greenish color.If now the substance of the paper extends without apparent interruption over the gray which lies under it, we think that glimmering through the greenish paper, and such an object be rose-red, in order to give white light.If, however, the grey spot has its limits so fixed that it appears to be an independent continuity with the greenish portion of the surface it as a gray object which lies on this surface."

The contrast-color may thus be made to disappear by placing in black the outlines of the gray scrap, or by placing above the tissue paper another gray scrap of the same degree of brightness, and comparing together the two grays.On neither of them does the contrast-color now appear.Hering shows clearly that this interpretation is incorrect, and that the disturbing factors are to be otherwise explained.In the first place, the experiment can be so arranged that we could not possibly be deceived into believing that we see the gray through a colored medium.