THE PERCEPTION OF SPACE. 213 



depth is as indefeasibly one of its attributes as its breadth. 

 We may artificially exaggerate this sensation of depth, 

 flise and look from the hill-top at the distant view ; repre- 

 sent to yourself as vividly as possible the distance of the 

 uttermost horizon ; and then with inverted head look at the 

 same. There will be a startling increase in the perspective, 

 a most sensible recession of the maximum distance ; and 

 as you raise the head you can actually see the horizon- 

 line again draw near.* 



Mind, I say nothing as yet about our estimate of the 

 - real ' amount of this depth or distance. I only want to 

 confirm its existence as a natural and inevitable optical 

 consort of the two other optical dimensions. The field of 

 view is always a volume-umt. Whatever be supjDosed to be 

 its absolute and ' real ' size, the relative sizes of its dimen- 

 sions are functions of each other. Indeed, it happens per- 

 haps most often that the breadth- and height-feeling take 

 their absolute measure from the depth-feeling. If we plunge 

 our head into a wash-basin, the felt nearness of the bottom 

 makes us feel the lateral expanse to be small. If, on the 

 contrary, we are on a mountain-top, the distance of the 

 horizon carries with it in our judgment a proportionate 



* What may be the physiological process connected with this increased 

 sensation of depth is hard to discover. It seems to have nothing to do with 

 the parts of the retina affected, since the mere inversion of the picture (by 

 mirrors, reflecting prisms, etc.), without inverting the head, does not seem 

 to bring it about ; nothing with sympathetic axial rotation of the eyes, 

 -which might enhance the perspective through exaggerated disparity ot 

 the two retinal images (see J. J. Milller, ' Raddrehung u. Tiefendimen- 

 siou,' Leipzig Acad. Berichte, 1875, psige 124), for one-eyed persons get 

 it as strongly as those with two eyes. I cannot hud it to be connected 

 with any alteration in the pupil or with any ascertainable strain in the 

 muscles of the eye, sympathizing with those of the body. The exaggera- 

 tion of distance is even greater when we throw the head over backwards 

 and contract our superior recti in getting the view, than when we bend 

 forward and contract the inferior recti. Making the eyes divert' e slightly 

 by weak prismatic glasses has no such effect. To me, and to all whom I 

 have asked to repeat the observation, the result is so marked that I do not 

 ■well understand how such an observer as Helmholtz, who has carefully 

 examined vision with inverted head, can have overlooked it. (See his 

 Phys. Optik, pp. 433, 723, 728. 772.) I cannot help thinking that anyone 

 who can explain the exaggeration of the depth-sensation in this case will 

 at the same time throw much light on its normal constitution. 



