The additions made in the course of the response are of several kinds. They may, perhaps, for our present purposes be spoken of without misunderstanding as images, or image-substitutes (see Chapter XVI). The eye, as is well known, is peculiar among our sense organs in that the receptor, the retina, is a part of the brain, instead of being a separate thing connected with the brain more or less remotely by a peripheral nerve. Moreover there are certain connections leading from other parts of the brain outwards to the retina as well as connections leading inwards. Thus there is some ground for supposing that through these outgoing connections actual retinal effects may accompany some visual images, which would thereby become much more like actual sensations than is the case with the other senses. However this may be, the process whereby an impression which, if interpreted in one way (e.g. by a person measuring the pigmented areas of a canvas), is correctly counted as a sign of a flat coloured surface, becomes, when differently interpreted, an intricately divided three-dimensional space—this process is one of the intervention of images of several kinds. The order of these interventions probably varies from case to case. Perhaps the most important of the images which come in to give depth, volume, solidity to the partly imagined and partly perceived ‘picture-space’ are those which are relicts of eye movements, kinæsthetic images of the convergence of the eyes and accommodation of the lenses according to the distance of the object contemplated. When, as it seems, we look past an object in a picture to some more distant object, seeming in so doing to change the focus of our eyes, we do not as a rule actually make any change. But certainly we feel as though we were focussing differently and as though the convergence were different. This felt difference which mainly gives the sense of greater distance is due to kinæsthetic imagery. Correspondingly the parts of the ‘picture-space’ upon which we seem to be focussing, upon which we _are_ imaginally focussing, become definite and distinct, and parts much nearer or much more distant become to some extent blurred and diffused. This effect is probably due to visual images, simulating the sensations which would normally ensue were we actually making a change of focus. The degree to which these last effects occur appears to differ very greatly from one person to another. Insufficient attention to the great variation in the means by which these images are involved by the painting is responsible for much bad criticism. Thus artists can commonly be found who are quite unable, when looking at paintings: in which the means employed are unlike their own, to apprehend forms over which less specialised persons find no difficulty. In general most visitors to Galleries pay too little attention to the fact that few pictures can be instantaneously apprehended, that even