=Effects of Contrast.=--The best-known examples of the way in which one nerve-current modifies another are the phenomena of what is known as 'simultaneous color-contrast.' Take a number of sheets of brightly and differently colored papers, lay on each of them a bit of one and the same kind of gray paper, then cover each sheet with some transparent white paper, which softens the look of both the gray paper and the colored ground. The gray patch will appear in each case tinged by the color _complementary_ to the ground; and so different will the several pieces appear that no observer, before raising the transparent paper, will believe them all cut out of the same gray. Helmholtz has interpreted these results as being due to a false application of an inveterate habit--that, namely, of making allowance for the color of the medium through which things are seen. The same _thing_, in the blue light of a clear sky, in the reddish-yellow light of a candle, in the dark brown light of a polished mahogany table which may reflect its image, is always judged of its own proper color, which the mind _adds_ out of its own knowledge to the appearance, thereby correcting the falsifying medium. In the cases of the papers, according to Helmholtz, the mind believes the color of the ground, subdued by the transparent paper, to be faintly spread _over_ the gray patch. But a patch to _look_ gray through such a colored film would have really to _be_ of the complementary color to the film. Therefore it _is_ of the complementary color, we think, and proceed to _see_ it of that color. This theory has been shown to be untenable by Hering. The discussion of the facts is too minute for recapitulation here, but suffice it to say that it proves the phenomenon to be physiological--a case of the way in which, when sensory nerve-currents run in together, the effect of each on consciousness is different from that which it would be if they ran in separately. '_Successive contrast_' differs from the simultaneous variety, and is supposed to be due to fatigue. The facts will be noticed under the head of 'after-images,' in the section on Vision. It must be borne in mind, however, that after-images from previous sensations may coexist with present sensations, and the two may modify each other just as coexisting sensational processes do. Other senses than sight show phenomena of contrast, but they are much less obvious, so I will not notice them here. We can now pass to a very brief survey of the various senses in detail. CHAPTER III. SIGHT. =The Eye's Structure= is described in all the books on anatomy. I will only mention the few points which concern the psychologist.[10] It is a flattish sphere formed by a tough [Illustration: FIG. 3.]