338 w. JAMES :

points on which the two images of an object fall does not within certain limits affect its seen singleness at all, but rather the distance at which it shall appear. Wheatstone made an observation, moreover, which subsequently became the bone of much hot contention, in which he strove to show that not only might disparate images fuse, but images on corresponding or identical points might be seen double. 1 I am unfortunately prevented by the weakness of my own eyes from experimenting enough to form a decided personal opinion on the matter. It seems to me, however, that the balance of evidence is against the Wheatstonian interpreta- tion, and that disparate points may fuse, without identical points for that reason ever giving double images. The two questions, " Can we see single with disparate points ? " and "Can we see double with identical points?" although at the first blush they may appear, as to Helmholtz they appear, to be but two modes of expressing the same in- quiry, are in reality distinct. The first may quite well be answered affirmatively and the second negatively. Add to this that the experiment quoted from Helmholtz above by no means always succeeds, but that many indi- viduals place their finger between the object and one of their eyes, oftenest the right. 2 Finally, observe that the identity- theory, with its Cyclopean starting-point for all lines of direction, gives by itself no ground for the distance on any Line at which an object shall appear, and has to be helped out in this respect by subsidiary hypotheses, which, in the hands of Hering and others, have become so complex as easily to fall a prey to critical attacks ; and it will soon seem tion. It seems a pity that England, leading off so brilliantly the modern epoch of this study, should so quickly have dropped out of the field. Almost all subsequent progress has been made in Germany, Holland and, longo intervallo, America. 1 This is no place to report this controversy, but a few bibliographic references may not be inappropriate. Wheatstone's own experiment is in section 12 of his memoir. In favour of his interpretation see Helmholtz, Phys. Opt., pp. 737-9 ; Wundt, Physiol PsychoL, 2te Ann., pp. 144 ; Nagel, tielien mit zwei Augen, pp. 78-82. Against Wheatstone see Volkmann, Arch. f. Ophth., v. 2-74 and Untersuchungen, p. 266 ; Hering, Beitrage zur Physiologic, 29-45, also in Hermann's Hdbch. d. Physiol., Bd. iii., 1 Th., p. 435 ; Aubert, Physiologic d. Netzhaut, p. 322 ; Schon, Archiv. /. Ophthal, xxiv., 1, p. 56-65 ; and Bonders, Ibid., xiii., 1, p. 15 and note. 2 When we see the finger the whole time, we usually put it in the line joining object and left eye if it be the left finger, joining object and right eye if it be the right finger. Microscopists, marksmen or persons one of whose eyes is much better than the other almost always refer directions to a single eye, as may be seen by the position of the shadow on their face when they point at a candle-flame.