Having thus cleared the ground, we naturally appeal to experiment; but unfortunately the first step that we take lands us in difficulties. It is found that, when all the cues above mentioned are ruled out, the estimation of distance is still possible; and many psychologists believe that it depends upon =kinæsthetic sensations= set up in and about the eye. Each eyeball is slung in its orbit upon six muscles; and the contraction of these muscles is, naturally, greater for convergence of the eyes upon near objects than for their convergence upon far; so that the _sensations of convergence_ seem fitted to play a part in the perception of distance. If only one eye is used, these sensations may be replaced by others, derived from the muscular system, within the eyeball, that adjusts or accommodates the lens for clear vision at different objective distances. The _sensations of accommodation_, though, in ordinary binocular vision they are entirely subordinate to the sensations of convergence, can nevertheless—within a lesser range of distances—play the same part in perception. Unfortunately, as was hinted just now, the results of these experiments are disputed; we shall come back to them, and to the possible rôle of the kinæsthetic sensations, later on. Meantime, what is to be said of the =eyes= themselves, and of the impression that a solid object, a tridimensional stimulus, makes upon them? If you hold up a closed book, back towards you, in the middle line of the face, and if you observe it alternately with the right and left eye, you will find that the two views do not tally; the left eye sees the back and the cover to the left, the right eye sees the back and the cover to the right. If you now make outline drawings of the two views, mount them upon a suitable card, and look at them through a =stereoscope=,—which, as you know, combines them into a single view,—lo! you have before you a solid book, the back near you, and the edges away in space. _It is as if the two eyes had reconciled their conflicting views, and the result were depth or solidity._