THE PERCEPTION OF SPACE, (ill.) 323

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 conscious- ness regarding it, without feeling that an indeterminate, palpitating, circling depth is as indefeasibly one of its attri- butes as its breadth. We may artificially exaggerate this sensation of depth. Rise and look from the hill-top at the distant view ; represent to yourself as vividly as possible the distance of the uttermost horizon ; and then loitli inverted head look at the same. There will be a startling increase in the perspective, a most sensible recession of the maximum distance ; arid as you raise the head you can actually see the horizon-line again draw near. 1 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-unit. Whatever be supposed 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 height and length in the mountain-chains that bound it to our view. But as aforesaid, let us not consider the question 1 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, &c.), 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 of the two retinal images (see J. J. Muller, "Kaddrehung u. Tiefendimen- sion," Sachs. Acad. Berichte, 1875, page 125), for one-eyed persons get it as strongly an those with two eyes. I cannot find it to be connected with any alteration in the pupil or with any ascertainable strain in the muscles of the eye, sympathising 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 diverge 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.