In the case of our second general question, that concerning the amount of change in the strength of solution required to produce a felt difference in the intensity of the taste sensation, various difficulties are involved. In the first place, there is no known way of measuring the intensity of that mode of stimulation which may be responsible for the excitation of the taste bud. In such cases as the sensibility to weight and light it is easily possible to measure the intensity of the stimulus in terms of pounds, candle power, or similar physical units. In the cases of temperature and sound the problem is much more difficult, since we do not know precisely what aspect of the stimulus should be used as indicating the intensity of the process at the point of stimulation. Taste and smell offer still greater difficulties, inasmuch as we do not know even with moderate certainty the real nature of the stimulation,—whether, for example, it be mechanical or chemical, or both. Consequently, although it is possible to state that in general a change in the stimulus intensity required to produce a sensation change which will be correctly reported in a certain percentage of the trials is 1% for light, 33% for sound, 5% for lifted weights, no such coefficients of change can even be suggested for taste and smell. Only the general statement can be made that, within limits, increased strength of solution means increased intensity of sensation. _Adaptation and Fatigue_ Curious phenomena in the case of all the senses are those known as adaptation and fatigue. It is a familiar experience that the illumination of a room which seems upon entrance to be yellowish quickly comes to appear merely light. After a few minutes wearing colored goggles the tinge they give to objects seems to disappear and we say we have become “adapted” to the color. “We become rapidly adapted to a constant stimulus so that we fail to notice the weight of our hats, the temperature of the room we are in, the odors of the subway.” Searching for spectacles which meanwhile perch upon the nose is the result of adaptation. A related phenomenon is the fact that a darkened room which on entrance seems perfectly black comes in time to show its contents as more or less clearly marked off from one another. Some adjustment—in the sense organs, perhaps, or perhaps in the brain centers—takes place in the presence of a constant stimulus. The general result of this adjustment is that the particular sense quality involved fades away: colors tend toward gray, pressures tend to disappear, temperatures tend toward a neutral point, and sounds become indifferent. This adjustment is not fatigue in the usual sense of the word. The area that has become adapted to a given pressure is still sensitive and will feel even a lighter weight if this be substituted for the one to which it has become adapted. It is adapted to a continuous stimulus, but it is sensitive to any change in the stimulus.