212 PSYCHOLOGY. 



study the facts closely loe soon find no such constant con- 

 nection heticeen either judgment and retinal modification, or 

 judgment and muscular modification, to exist. The judgment 

 seems to result from the combination of retinal, muscular 

 and intellectual factors with each other ; and any one of 

 them may occasionally overpower the rest in a way which 

 seems to leave the matter subject to no simple law. 



The scientific study of the subject, if we omit Descartes, 

 began with Berkeley, and the particular perception he 

 analyzed in his New Theory of Yision was that of distance 

 or depth. Starting with the physical assumption that a 

 difference in the distance of a point can make no difference 

 in the nature of its retinal image, since "distance being a 

 line directed endwise to the eye, it projects only one point 

 in the fund of the eye — which point remains invariably the 

 same, whether the distance be longer or shorter," he con- 

 cluded that distance could not possibly be a visual sensation, 

 but must be an intellectual * suggestion ' from ' custom ' 

 of some non-visual experience. According to Berkeley this 

 experience was tactile. His whole treatment of the subject 

 was excessively vague, — no shame to him, as a breaker of 

 fresh ground, — but as it has been adopted and enthusiasti- 

 ally hugged in all its vagueness by nearly the whole line of 

 British psychologists who have succeeded him, it will be 

 well for us to begin our study of vision by refuting his 

 notion that depth cannot possibly be perceived in terms of 

 purely visual feeling. 



The Third Dimension. 



Berkeleyans unanimously assume that no retinal sensa- 

 tion can primitively be of volume ; if it be of extension at 

 all (which they are barely disposed to admit), it can be only 

 of two-, not of three-, dimensional extension. At the begin- 

 ning of the present chapter we denied this, and adduced 

 facts to show that all objects of sensation are voluminous 

 in three dimensions (cf. p. 136 ff.). It is impossible to lie 

 on one's back on a hill, to let the empty abyss of blue fill 

 one's whole visual field, and to sink deeper and deeper into 

 the merely sensational mode of consciousness regarding it, 

 without feeling that an indeterminate, palpitating, circling 



