Page:Popular Science Monthly Volume 51.djvu/109

Rh "feels" heat is, from a physiological point of view, an indirect question. Physiologically, the nerves of the skin may respond to heat by some chemical process; that they do so respond may be inferred on the hypothesis of a correspondence between the occurrence of a sensation of heat and the action of the nerve. The direct question is one of psychology; it is asked by physiology for its own purposes, and the psychological data are collected as long as they are of use in this way. Physiology, however, is "physics and chemistry of the body," and as soon as psychological data cease to afford physical deductions the interest of the physiologist generally ceases. The study of the psychology of sensation and action, however, has formed and still forms an important portion of physiology.

Historically considered, the study of the sensations of the skin received its first great impulse from Ernst Heinrich Weber's monograph, Tastsinn und Gemeingefühle. This has been followed by the work of a host of investigators from the laboratories of Ludwig, Du Bois-Reymond, and their pupils.

The physiology of the eye originated much of the psychology of sight. Concerning the functions of the optical system, physiology can scarcely be said to have gone beyond the dioptrics of the eye. Nearly all further knowledge consists of deductions from the mental experiences of the subject. For example, physiology knows almost nothing concerning the functions of the retina. Psychologically, however, the color sensations and their combinations can be accurately measured. It is true that the investigations of color vision have been and are mainly carried out by physiologists and physicists; but the point of view has become primarily a purely psychological one. This is strikingly exemplified in the researches of König, from which physiological deductions are practically excluded. For the various other phenomena, such as those of the optical illusions, of monocular and of binocular space, we have at present no hope of anything beyond a psychological knowledge, and the investigations of Hering, Helmholtz, and others can be regarded as direct contributions to psychology.

There is a third science whose influence is to-day the strongest of all. Physics is theoretically the co-ordinate science to psychology. Every direct experience has an objective, or physical, and a subjective, or psychical, side. Again, the fundamental science of Nature is physics, that of Mind is psychology. Practically, however, psychology receives from the most powerful science