266 PSYCHOLOGY. 



We may then sum up our study of illusions by saying that 

 they in no ivise undermine our vieiv that every spatial determi- 

 nation of things is originally given in the shape of a sensation 

 of the eyes. Thej only show liow very potent certain 

 imagined sensations of the eyes may become. 



These sensations, so far as they bring definite forms to 

 the mind, appear to be retinal exclusively. The move- 

 ments of the eyeballs play a great j)art in educating our 

 perception, it is true ; but they have nothing to do with 

 constituting any one feeling of form. Their function is 

 limited to exciting the various feelings of form, by tracing 

 retinal streaks ; and to comparing them, and measuring them 

 off against each other, by applying different parts of the 

 retinal surface to the same objective thing. Helmholtz's 

 analysis of the facts of our ' measurement of the field of vieiv ' 

 is, bating a lapse or two, masterly, and seems to prove that 

 the movements of the eye have had some part in bringing 

 our sense of retinal equivalencies about — equivalencies, mind, 

 of different retinal forms and sizes, not forms and sizes 

 themselves. Superposition is the way in which the eye- 

 movements accomplish this result. An object traces the 

 line AB on a peripheral tract of the retina. Quickly we 

 move the eye so that the same object traces the line ah on 

 a central tract. Forthwith, to our mind, AB and ah are 

 judged equivalent. But, as Helmholtz admits, the equiv- 

 alence-judgment is independent of the way in which we 

 may feel the form and length of the several retinal pic- 

 tures themselves : 



"The retina is like a pair of compasses, whose points we apply in 

 succession to the ends of several lines to see whether they agree or not in 

 length. All we need know meanwhile about the compasses is that the 

 distance of their points remains unchanged. What that distance is, and 

 what is the shape of the compasses, is a matter of no account."* 



gent or not, for their divergence differs in individuals and often in one in- 

 dividual at diverse times — precludes its being due to the mere habitual 

 falling off of the image of one objective line on both. Le Conte, e.g., 

 measures their position down to a sixth of a degree, others to tenths. This 

 indicates an organic identity in the sensations of the two retinse, which the 

 experience of median perspective horizontals may roughly have agreed 

 with, but hardly can have engendered. Wundt explains the divergence as 

 usual, by the Innervationsgefilhl {op. cit. ii. 99 ff.). 

 * Physiol. Optik, p. 547. 



