THE PERCEPTION OF SPACE. 227 



points on wliicli the two images of au object fall does not 

 within certain limits affect its seen singleness at all, but 

 Tather the distance at which it shall appear. Wheatstone 

 made an observation, moreover, which subsequently bec£!,me 

 the bone of much hot contention, in which he strove to 

 show that not only might disparate images fuse, but im- 

 ages on corresponding or identical points might be seen 

 •double.* 



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 

 interpretation, and that disparate points may fuse, without 

 identical points for that reason ever gi^dng 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 Helm- 

 holtz they appear, to be but two modes of expressing the 

 same inquiry, 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 i^lace their finger between the object and one of 

 their eyes, oftenest the right ; t finally, observe that the 



epoch of this study, should so quickly have dropped out of the field. 

 Almost all subsequent piogie.ss has been made in Germany, Holland, and, 

 longo intervallo, America. 



* This is no place to report this controversy, but a few bibliographic 

 references may not be inappropriate. Wheatstone's own experiment is in 

 section 13 of his memoir. In favor of his interpretation see Helmholtz, 

 Phys. Opt., pp. 737-9 ; Wundt, Physiol. Psychol., 2teAuti. p. 144; Nagel, 

 Sehen mit zwei Augen, pp. 78-82. Against Wheatstone see Volkmann, 

 Arch. f. Ophth., v. 2-74, and Untersuchungen, p. 266 ; Hering, Beitragezur 

 Physiologic, 29-45, also in Hermann's Hdbch. d. Physiol., Bd. iii. 1 Tb. 

 p. 435 ; Aubert, Physiologic d. Netzhaut, p. 322 ; Schon, Archiv f. Ophthal., 

 XXIV. 1. pp. 56-65 ; and Bonders, ibid, xni 1. p. 15 and note. 



f 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-flajne. 



