530 w. JAMES :

a human face, the correct inclination of an angle or sweep of a curve, or the distance of two lines. Our mind will be like a polyhedron, with facets, attitudes of perception in which it can most easily rest. These are worn upon it by habitual objects, and from one of these it can pass only by tumbling over into another. Hering has well accounted for the sensationally vivid character of these habitually reproduced forms. He says, after reminding us that every visual sensation is correlated to a physical process in the nervous apparatus : " If this psych ophysical process is aroused, as usually happens, by light- rays impinging on the retina, its form depends not only on the nature of these rays, but on the constitution of the entire nervous apparatus which is connected with the organ of vision, and on the state in which it finds- itself. The same stimulus may excite widely different sensations according to this state. " The constitution of the nervous apparatus depends naturally in part upon innate predisposition ; but the ensemble of effects wrought by stimuli upon it in the course of life, whether these come through the eyes or from elsewhere, is a co-factor of its development. To express it otherwise, in- voluntary and voluntary experience and exercise assist in determining the material structure of the nervous organ of vision, and hence the ways in which it may react on a retinal image as an outward stimulus. That ex- perience and exercise should be possible at all in vision is a consequence of the reproductive power, or memory, of its nerve-substance. Every particu- lar activity of the organ makes it more suited to a repetition of the same; ever slighter touches are required to make the repetition occur. The organ habituates itself to the repeated activity. . . . " Suppose now that, in the first experience of a complex sensation pro- duced by a particular retinal image, certain portions were made the special objects of attention. In a repetition of the sensible experience it will happen that notwithstanding the identity of the outward stimulus these portions will be more easily and strongly reproduced ; and when this happens a hundred times the inequality with which the various constituents of the complex sensation appeal to consciousness grows ever greater. " Now in the present state of our knowledge we cannot assert that in both the first and the last occurrence of the retinal image in question the same pure sensation is provoked, but that the mind interprets it differently the last time in consequence of experience ; for the only given things we know are on the one hand the retinal image which is both times the same, and 011 the other the mental percept which is both times different ; of a third thing, such as a pure sensation, interpolated between image and percept, we know nothing. We ought therefore, if we wish to avoid hypo- theses, simply to say that the nervous apparatus reacts the last time dif- ferently from the first, and gives us in consequence a different group of sensations. " But not only by repetition of the same retinal image, but by that of similar ones, will the law obtain. Portions of the image common to the successive experiences will awaken, as it were, a stronger echo in the nervous apparatus than other portions. Hence it results that reproduction is usually elective: the more strongly reverberating parts of the picture yield stronger feelings than the rest. This may result in the latter being quite overlooked and, as it were, eliminated from perception. It may even