THE PERCEPTION OF SPACE. 261 



after reminding ns that every visual sensation is correlated 

 to a physical process in the nervous apparatus : 



" If this psychophysical process is aroused, as usually happens, by 

 light-rays impinging on the retina, its form depends not only on the na- 

 ture of these rays, but on the constitution of the entire nervous appa- 

 ratus 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, involuntary 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 experience and exercise should be possible at all in 

 vision is a consequence of the reproductive power, or memory, of its 

 . nerve-substance. Every particular 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 

 produced 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 on 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 hypotheses, simply to say that the nervous apparatus 

 reacts the last time differently from the first, and gives us in conse- 

 quence 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 repro- 

 duction is usually elective : the more strongly reverberating parts of the 

 picture yield stronger feelings than the rest. This may result in the 



