(1) Salt and sour contrast: the sour induced by salt being clearer and stronger than the salt induced by sour. (2) Sweet and sour contrast: the sweet induced by sour being clearer and stronger than the sour induced by sweet. (3) Salt and sweet contrast: the sweet induced by salt being clearer and stronger than the salt induced by sweet. (4) Bitter shows no contrast at all. (5) The order of qualities, as regards ease of induction, is sweet, sour, salt, bitter. _After Images of Taste_ Suggested by the phenomena of contrast are the somewhat related facts of after sensations or after images, as they are sometimes called. When one looks for a moment at a candle or other source of light and then quickly extinguishes it or looks away from it, one still continues to see, for a time, a luminous form, which may persist for a considerable time after the removal of the stimulus. In such a case the color and brightness of this after image may be the same as those of the original object, and the after image is hence said to be _positive_. Under certain conditions the colors of the after image are complementary to those of the original and the brightness relations of the various parts are reversed. The after image is then said to be _negative_. Or if after looking at a colored object one transfers his gaze to a gray expanse there appears upon this gray field an outline of the original object, with colors which are complementary or antagonistic to those of the original. After sensations of pressure arising under special conditions have been described, and positive after effects of warm and cold stimuli seem also to be demonstrable. Even after sensations of sound, somewhat weak, transitory, and by no means easily detected, have been described. In all these cases except vision the after sensations are of the positive type only. In the case of taste, and of smell also, it is difficult to investigate the presence of such after sensations, inasmuch as it is by no means easy to be sure that some trace of the stimulus does not remain in or near the sense organ. An experience reported as a positive after sensation might easily enough represent only the effect of persistent stimulation by these traces of the substance. At least one investigator is convinced that in his observations of taste experiences “the sensation continued after the tongue was so carefully dried off that no particles of the tastable substance were left.” Similarly, experiences of tastes being “left in the mouth” are very common. But our inadequate control over the disposition of the sapid substance and the complicated chemical relation which exists between various substances and between some substances and the natural juices secreted in the vicinity of the taste organ makes it impossible to assert with certainty either the presence or the absence of after sensations of taste. _The Schema of Taste Relations_