Another frequent cause of the apparent absence of contrast is the presence of narrow dark intermediate fields, such as are formed by _bordering a field with black lines, or by the shaded contours of objects_. When such fields interfere with the contrast, it is because black and white can absorb much color without themselves becoming clearly colored; and because such lines separate other fields too far for them to distinctly influence one another. Even weak objective differences in color may be made imperceptible by such means. A third case where contrast does not clearly appear is where the _color of the contrasting fields is too weak or too intense_, or where there is _much difference in brightness between the two fields_. In the latter case, as can easily be shown, it is the contrast of brightness which interferes with the color-contrast and makes it imperceptible. For this reason contrast shows best between fields of about equal brightness. But the intensity of the color must not be too great, for then its very darkness necessitates a dark contrasting field which is too absorbent of induced color to allow the contrast to appear strongly. The case is similar if the fields are too light. _To obtain the best contrast-effects, therefore, the contracting fields should be near together, should not be separated by shadows or black lines, should be of homogeneous texture, and should be of about equal brightness and medium intensity of color._ Such conditions do not often occur naturally, the disturbing influences being present in case of almost all ordinary objects, thus making the effects of contrast far less evident. To eliminate these disturbances and to produce the conditions most favorable for the appearance of good contrast-effects, various experiments have been devised, which will be explained in comparing the rival theories of explanation. * * * * * There are _two theories--the psychological and the physiological_--which attempt to explain the phenomena of contrast.