357 
of Edinburgh, Session 1870-71. 
remain distinct objects of vision. It may however be mentioned, 
by the way, that either of the cards may be recalled to sight by 
the simple act of moving it two or three times backwards and 
forwards, and thus exciting the nerve and arousing the attention ; 
hut this in no degree impairs the superior brightness of the over- 
lapping parts. 
Such are the facts, but what is the cause of the increased bright- 
ness where the cards appear to overlap, and what is the cause of 
the apparent overlapping where corresponding points of the retinae 
are excited by objects in reality apart? I am not aware of any 
writer having distinctly laid before us a specific physical cause 
accounting for these several phenomena. It appears to me that 
they clearly point to an anatomical cause. 
A great many writers have attributed single vision to habit. 
Dr Smith in his optics attributes single vision to this cause. Dr 
Carpenter also seems to take this view. He says (“Physiology,” p. 
705), “ A condition of single vision seems to be that the two 
images of the object should fall on parts of the retinae accustomed 
to act in concert, and habit appears to be the chief means by which 
this conformity is produced.” Dr Reid, in his “ Inquiry into the 
Human Mind,” states that he has devoted thirty years to the study 
of the subject, and he accepts it as a mystery which cannot be 
explained. Sir Wm. Hamilton attempts no explanation. Neither 
does Sir D. Brewster in his famous controversy with Professor 
Wheatstone attempt any explanation. Buffon thinks we first see 
objects double and inverted, and that we correct this judgment by 
experience. Blanville, Grassendus, Porta, Tacquet, and Grail, main- 
tain that we see with only one eye at a time. 
Perhaps the majority of writers have looked no deeper than 
the surface of the retina, and have been content to state the 
phenomena as depending on an inscrutable property of that 
sensitive membrane, or simply as a law of our being : even as they, 
with quite as little ingenuity, and with less excuse, attribute our 
sense of visual direction to an inscrutable property of the retina. 
Some anatomists have, however, supposed that the decussation of the 
optic nerves might explain the phenomena. Dr Wollaston, from a 
peculiar occasional disorder in his vision, suggested that there was 
a crossing of the fibres from the inner parts of either retina to the 
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VOL. VII. 
