(c) As Monocular. Under this head we have to consider conditions which remain operative even when only one eye is used. These conditions are mainly of an auxiliary kind. They do not in the first instance enter into the constitution of the perception of depth; but when once it has been otherwise formed, they reproduce it by association. There is only one monocular experience which appears capable of directly constituting the perception of depth. This is the varying accommodation of the lens by which distinct vision is secured at varying distances of the object from the eye. The importance of this factor seems small in comparison with the part played by movements of convergence of the two eyes. But it does seem to supply within limits the sort of combination of passive and active sight required for perception of the third dimension. The nearer the object, the more convex must the surface of the lens be, if a distinct image is to be focussed on the retina ; and the more remote the object, the flatter must it be. If the lens is too convex or too flat, what are called "circles of diffusion" occur on the retina, and the image is indistinct. Thus, in fixing the eye successively on more and more distant points of a line, the lens will be accommodated at any moment for the point looked at and yield a distinct image of this. Points nearer or more remote will produce progressively more indistinct and diffused impressions, the greater is their distance from the fixationpoint. As the glance moves to and fro along the line, the indistinct becomes progressively distinct, and vice versa. Thus we have a total experience analogous to that accompanying increasing or decreasing convergence of the two eyes. The result in this case also is a perception of position and distance in the third dimension. Here too, muscular sensations probably contribute to the result. The adjustment of the lens depends upon a muscle which by its contraction slackens a ligament to which the lens is attached. When the ligament is slackened, the lens, owing to its own elasticity, bulges and becomes more convex. There are distinct motor sensations accompanying this process of motor accommodation. As in the case of binocular vision, a series of motor experiences, accompanying movement of the eyeball, are conjoined with a series of optical experiences, due to the varying disparateness of retinal impressions; so, in monocular vision, a series of motor experiences accompanying accommodation of the lens, is conjoined with a series of optical experiences, due to varying distinctness and diffusion of retinal impressions.
We have now to turn to another class of conditions operative in monocular as well as binocular vision, which may be called secondary or associative. They would not of themselves produce the perception of depth, but their variations are so intimately conjoined in experience, with varying distance and position in the third dimension, that a process of complication has taken place, so that now they produce depthperceptions as immediately and distinctly as if they were themselves contributive factors in the apprehension of the third dimension. All conditions of this kind, and no others, are used by the artist in producing the perception of depth in pictures. It should be noted that depth and solid figure as they appear in the work of an artist are actually perceived. We do not in looking at a picture merely see combinations of lines on a plane surface, which call up mental images of objects in the third dimension. On the contrary, the drawing is seen in the third dimension from the outset. The artifices used by the painter do not merely suggest ideal representations of depth, but actually produce the perception of depth. This perception is doubtless different in nature from that which is produced by the actual object, but none the less it truly belongs to the perceptual and not to the ideational consciousness. Among these associative conditions we may refer first to the variation in the area covered by the retinal impression of an object, according to its varying distance from the eye. This might in itself produce merely a corresponding variation in the apparent magnitude of the thing seen: indeed, by concentrating our attention on the visual sensation, as such, we can detect changes in the size of the object according to its changing distance: but normally our attention is otherwise directed. The real object does not vary in size; and what we are interested in is the real object and not our own sensations. We accordingly tend to ignore these differences in the extensity of retinal sensation except in so far as they mark different distances in the third dimension. This is of course only possible if the actual size of the objects is otherwise known by previous experiences in which we have moved close up to them, so that the retinal image has passed through a series of changes giving place at last to that image which accompanies and guides actual contact. To appreciate the full importance of this condition, we must remember that all the objects within the field of view and the different parts of the same object produce retinal impressions varying in extent in a systematic and regular way, according to their distance from the eye. The imitation of this systematic diminution of size with increasing distance is in the hands of the artist a most potent means of producing stereoscopic effect. Where the varying distance of an object is fixed by other means, the extent of the retinal impression mainly determines perception of magnitude. This is well seen in the case of afterimages. " Produce an afterimage o the sun and look at your fingertip; it will be smaller than your nail. Project it on the table, and it will be as big as a strawberry; on the wall, as large as a plate; on yonder mountain, bigger than a house. And yet it is an unchanged retinal impression."* An actual object producing a retinal excitation of the same extent would vary in size according to its distance. Hence the afterimage appears of different sizes, when it is perceived at different distances. But the actual retinal sensation is in all cases the same.
*James, Psychology, vol. ii., p. 231.