RELATION OF BINOCULAR VISION TO STEREOSCOPY.
Reference being made to Fig. S18, let the observer's two eyes be represented at n and it, and an object under immediate attention at A, to which the axes of the eyes are directed. Whilst A is the object of particular attention, the lenses of the eyes, together with their supplementary humours, will he accommodated for dis tinct vision of A, a picture of which will fall upon the centre of the retina at A' in both eyes, the result being a distinctly projected image of A upon each retina, producing a single impression in the brain, i.e. the mind will be conscious of a single object standing before the eyes at A. An object at c, situated in the same plane with A, will also be singly observed, because its image in each eye falls upon c', a point at an equal distance to the left of the axes in both eyes. This object, however, though sPen singly and clearly, is not a predominating object in the mind, because it is only seen obliquely, and light therefrom reaches a part of the retina slightly removed from the centre. With the attention still
riveted upon A, an object nearer to the eyes at B will not only be indistinctly seen, because the lenses of the eyes are not accommodated for so near a plane, but its indistinct images upon the retina \Vill not fall on corresponding points ; thus, in the left eye it will be on the left of the axis, while in the right eye it will be on the right. It will therefore appear double. Let a finger be held in front of the face, and about twelve inches therefrom, mean while looking intently at an object on the other side of the room. While the dis tant object is perceived as a single one, the finger will appear double. If now, without moving the head, the attention is turned more especially to the finger, the latter, which hitherto appeared double, will now be seen as a single finger, while the distant object first claiming attention is seen double.