Sensory adaptation is a change that occurs in other senses also, but it is so much more important in the sense of sight than elsewhere that it may best be considered here. The stimulus continues, the sensation ceases or diminishes--that is the most striking form of sensory adaptation. Continued action of the same stimulus puts the sense into such a condition that it responds differently from at first, and usually more weakly. It is much like fatigue, but it often is more positive and beneficial than fatigue.

The sense of smell is very subject to adaptation. On first entering a room you clearly sense an odor that you can no longer get after staying there for some time. This adaptation to one odor does not prevent your sensing quite different odors. Taste shows less adaptation than smell, but all are familiar with the decline in sweet sensation that comes with continued eating of sweets.

All of the cutaneous senses except that for pain are much subject to adaptation. Continued steady pressure gives a sensation that declines rapidly and after a time ceases altogether. The temperature sense is usually adapted to the temperature of the skin, which therefore feels neither warm nor cool. If the temperature of the skin is raised from its usual level of about 70 degrees Fahrenheit to 80 or 86, this temperature at first gives the sensation of warmth, but after a time it gives no temperature sensation at all; the warmth sense has become adapted to the temperature of 80 degrees; and now a temperature of 70 will give the sensation of cool. {225} Hold one hand in water at 80 and the other in water at 66, and when both have become adapted to these respective temperatures, plunge them together into water at 70; and you will find this last to feel cool to the warm-adapted hand and warm to the cool-adapted. There are limits to this power of adaptation.

The muscle sense seems to become adapted to any fixed position of a limb, so that, after the limb has remained motionless for some time, you cannot tell in what position it is; to find out, you have only to move it the least bit, which will excite both the muscle sense and the cutaneous pressure sense. The sense of head rotation is adaptable, in that a rotation which is keenly sensed at the start ceases to be felt as it continues; but here it is not the sense cells that become adapted, but the back flow that ceases, as will soon be explained.

To come now to the sense of sight, we have light adaptation, dark adaptation, and color adaptation. Go into a dark room, and at first all seems black, but by degrees--provided there is a little light filtering into the room--you begin to see, for your retina is becoming dark-adapted. Now go out into a bright place, and at first you are "blinded", but you quickly "get used" to the bright illumination and see objects much more distinctly than at first; for your eye has now become light-adapted. Remain for some time in a room illuminated by a colored light (as the yellowish light of most artificial illuminants), and by degrees the color sensation bleaches out so that the light appears nearly white.

Dark adaptation is equivalent to sensitizing the retina for faint light. Photographic plates can be made of more or less sensitiveness for use with different illuminations; but the retina automatically alters its sensitivity to fit the illumination to which it is exposed.{226}