Page:Optics.djvu/133

 148. The different means by which, according to Harris, we are enabled to correct our observations of distances, are
 * The change of conformation in the eye necessary for the distinct perception of objects at different distances.
 * The inclination of the axes of the two eyes, when directed to the same object.
 * The length of the ground plane, or the number of interveneing parts perceived in it.
 * The different appearances of known objects at different distances, or the known magnitudes of their least visible parts.
 * Different degrees of brightness and change of colour.

With regard to the two first of these, it is allowed on all hands that they can be available only for very short distances. Harris, in his concluding observation on them, says: "I think it is very manifest that with both our eyes we can distinguish, pretty accurately, the places of objects that are not above 5 or 6 feet distance from us. And indeed it seems necessary that we should have something within ourselves, or some means that never forsake us, whereby we might unerringly judge of distances so very near us: otherwise we might be frequently in danger of our lives, without perceiving it; as well as subject to perpetual mistakes, concerning objects so near as within the reach of our arm."

He had just observed as an illustration of this: "If I advance within 4 or 5 feet of an image projected before a concave speculum, I can define its place very precisely; and the image itself, though much smaller than the object, will appear very perfect and continue still at the same place, as I advance nigher: but on my retreating farther back than my first station, I begin to be less certain of its place, and the mistake lies on supposing it farther