THE PERCEPTION OF SPACE. 217 



two other dimensions seem to follow. If, to use tlie former 

 instance, I look close into a wash-basin, the lateral extent 

 of the field shrinks proportionately to its nearness. If I 

 look from a mountain, the things seen are vast in height 

 and breadth, in proportion to the farness of the horizon. 

 But ivhen ive ask what changes in the eye determine hoiv great 

 this maximum feeling of depth or distance (which is undoubt- 

 edly felt as a unitary vastness) shall be, we find ourselves 

 unable to point to any one of them as being its absolutely regular 

 concomitant. Convergence, accommodation, double and 

 disparate images, differences in the parallactic displacement 

 when Ave move our head, faintness of tint, dimness of out- 

 line, and smaliness of the retinal image of objects named 

 and known, are all processes that have something to do with 

 the perception of ' far ' and of ' near ' ; but the effect of 

 each and any one of them in determining such a perception 

 at one moment may at another moment be reversed by the 

 presence of some other sensible quality in the object, that 

 makes us, evidently by reminding us of past experience, 

 judge it to be at a different distance and of another shape. 

 If we paint the inside of a pasteboard-mask like the out- 

 side, and look at it with one eye, the accommodation- and 

 parallax-feelings are there, but fail to make us see it hollow, 

 as it is. Our mental knowledge of the fact that human 

 faces are always convex overpowers them, and we directly 

 perceive the nose to be nearer to us than the cheek instead 

 of farther of. 



The other organic tokens of farness and nearness are 

 proved by similar experiments (of which we shall ere long 

 speak more in detail) to have an equally fluctuating import. 

 They lose all their value whenever the collateral circum- 

 stances favor a strong intellectual conviction that the object 

 presented to the gaze is improbable — cannot be either ivhat 

 or where they would make us perceive it to be. 



Now the query immediately arises : Can the feelings of 

 these processes in the eye, since they are so easily neutralized and 

 reversed by intellectual suggestions, ever have been direct sensa- 

 tions of distance at all ? Ought we not rather to assume, 

 since the distances which we see in spite of them are con- 

 clusions from past experience, that the distances which we 



