of white mingles itself with that of red and makes the red color of the paper look a little grayish. The success of the experiment with the pure colors of the spectrum, which contain no white, is easily accounted for by the explanation just given.
All these phenomena are cases of what is called successive contrast, because we look in succession from one surface to another. When colored surfaces are placed near each other and compared in a natural manner, successive contrast plays an important part, and the appearance of the colors is more or less modified according to its laws. If we attempt to confine our attention to only one of the colored surfaces, this still holds good, for the eye involuntarily wanders to the other, and to prevent this requires a good deal of careful practice, for fixed vision is quite opposed to our natural habit. It follows from this that, in the natural use of the eye, the negative images, although present to some extent, are not sharp and distinct, and hence usually remain unobserved by persons not trained to observations of this character. Nevertheless these images modify to a considerable extent the appearances of colored surfaces placed near each other, and the changes of hue are visible enough to the most uneducated eye.
One of the most common cases belonging here is represented in Fig. 8. We have a gray pattern traced on a green ground; the
tracery, however, will not appear pure gray, but tinged with a color complementary to that of the ground; that is, reddish. We can, of course, substitute for the green any other bright color, and it will always be found that the gray pattern is more or less tinged with the