To this theory it may, perhaps, be objected, that, in the act of seeing, the motion of the head is too rapid to effect any material difference; but it will be found, on examining this subject attentively, that the object is seen in a new point of view, from the instant the head is moved, because the rays no longer meet at the same centre; and, therefore, the effect of such vision on the mind, is rather a renewal in succession of similar ideas, than the same single idea simultaneously excited: and this difference may be compared to that between seeing a landscape reflected in a mirror at rest, and the same landscape when the mirror has been removed from its original position*.
*[Perhaps this difference may be more familiarly explained by observing, that, when a lark ascends in the air, we have no difficulty in keeping the bird in sight so long as we continue our head in the first position; but from the moment the head is moved, we have to search for the object again, and often in vain, through the vast expanse of sky.]
