That we, in reality, see all objects double, is also evident; for example, if we hold up a finger, and look with the right eye at an object, it will appear against one particular part of the room, shutting that and looking with the left, it will seem to be on a different part, and if we open both eyes, the object will appear to be placed between the two extremes. But the truth is, the image of the object is formed in both eyes, one of which appears to the right, and the other to the left; and it is from the habit of touching that we suppose we see but one image placed between them both. From which it is clear that we see all objects double, although our imagination forms them single; and that, in fact, we see things where they are not, notwithstanding we have a pretty exact idea of their situation and position; and thus it is that till the sense of feeling has corrected the errors of sight, if instead of two eyes, we had an hundred, we should still fancy the objects single, although they were multiplied an hundred times. In each eye, therefore, is formed a separate image of every object; and when the two images strike the correspondent parts of the retina, that is, the parts which are always affected at the same time, the object appears single: but, when the images strike the parts of the retina, which are not usually affected together, then it appears double, because we are not habitual to this unusual sensation, and are then somewhat in the situation of infants just beginning to exert the faculty of vision. M. Chesselden relates the case of a man, who, in consequence of a blow on the head, became squint-eyed, and saw objects double for a long time: but who was at length enabled, by slow and gradual steps, to see them singly as he had formerly done, notwithstanding the squinting remained. Is not this a proof, still more evident, that in reality we see things double, and that it is by habit alone we conceive them to be single? Should it be asked why children require less time, in order to see things single, than persons more advanced in years, whose eyes may have been affected by accident? it might be answered, that the sensations of children, being unopposed by any contradictory habit, these errors are rectified with ease; but that persons who have for many years seen objects single, because they affected the two correspondent parts of the retina, and who now see them double, labour under the disadvantage of having a contrary habit to oppose, and must therefore be a considerable time before it is entirely obliterated.