S20 PSYCHOLOGY. 



now in advance that we shall find the means to be nothing 

 more or less than association — tlie suggestion to the mind of 

 optical objects not actually present, but more habitually asso- 

 ciated with the 'collateral circumstances' than the sensa- 

 tion which they now displace and being imagined now with 

 a quasi-hallucinatory strength. But before this conclu- 

 sion emerges, it will be necessary to have reviewed the 

 most important facts of optical space-perception, in relation 

 to the organic conditions on which they depend. Readers 

 acquainted with German optics will excuse what is already 

 familiar to them in the following section.* 



* Before embarking on this new topic it will be well to shelve, once for 

 all, the problem of what is the physiological process that underlies the 

 distance-feeling, Since one-eyed people have it, and are inferior to the 

 two-eyed only in measuring its gradations, it can have no exclusive connec- 

 tion with the double and disparate images produced by binocular parallax. 

 Since people with closed eyes, looking at an after-image, do not usually 

 see it draw near or recede with varying convergence, it cannot be simply 

 constituted by the convergence-feeling. For the same reason it would 

 appear non-identical with the feeling of accommodation. The differ- 

 ences of apparent parallactic movement between far and near objects as 

 we move our head cannot constitute the distance-sensation, for such dif- 

 ferences may be easily reproduced experimentally (in the movements of 

 visible spots against a background) without engendering any illusion of per- 

 spective. Finally, it is obvious that visible faintness, dimness, and small- 

 ness are not pe7' se the feeling of visible distance, however much in the 

 case of well-known objects they may serve as signs to suggest it. 



A certain maximum distance-value, however, being given to the field of 

 view of the moment, whatever it be, the feelings that accompany the pro- 

 cesses just enumerated become so many local signs of the gradation of 

 distances within this maximum depth. They help us to subdivide and 

 measure it. Itself, however, is felt as a unit, a total distance-value, deter- 

 mining the vastness of the whole field of view, which accordingly appears 

 as an abyss of a certain volume. And the question still persists, what 

 neural process is it that underlies the sense of this distance-value ? 



Hering, who has tried to explain the gradations within it by the inter- 

 action of certain native distance-values belonging to each point of the two 

 retinae, seems willing to admit that the absolute scale of the space-volume 

 ■within which the natively fixed relative distances shall appear is not fixed, 

 but determined each time by ' experience in the widest sense of the word * 

 (JBeitrdge, p. 344). What he calls the Kernpunkt of this space-volume is 

 the point we are momentarily fixating. The absolute scale of the whole 

 volume depends on the absolute distance at which this Kernpunkt is judged 

 to lie from the person of the looker. "By an alteration of the localization 

 of the Kernpunkt, the inner relations of the seen space are nowise altered ; 

 this space in its totality is as a fixed unit, so to speak, displaced with re- 



