W. B. Rogers on Binocular Vision. 87 
servations in binocular vision, he is I think entitled to the credit 
of having first perceived a chief condition of these phenomena 
ut the law of the apparent place of objects binocularly com- 
bined is so fundamentally important as to call for other and bet- 
ter modes of proof. One such, remarkable for its completeness, 
has been described by Sir David Brewster (Phil. Mag., 3d Series, 
Vol. 24), and I may be permitted to add the following simple ex- 
periments for the same Js which I have found very satisfac- 
tory and easy of executior 
, fess — | 
f SERS OEE” ai 
a 
rs 
On alight strip of wood about 3 feet long au i; inches wide, 
two lines are to be drawn diverging from nd at such an an- 
on 
gle that their terminations at the other onde 1all be a little nearer 
one another than the pupils of the two eyes (fig. 1). Three 
steel pins each about 24 inches high and surmounted by round 
heads }th inch in di fc then be fixed perpendicularly in 
the hem a 
eye, and by b in the left. If now we direct our attention to the 
remote pin r we see what appears to be another pin thicker and 
taller than r, coinciding with it or very near it. This is the 
binocular resultant of a and b as seen by the two eyes con- 
verge 
To obtain this effect in the most satisfactory manner the un- 
practised observer will find it useful to conceal the surface of the 
while directing the eyes steadily towards r to perceive the 
resultant image of a and b, we gently peg the instrument in 
its plane around a point midway in a b, we see the remote pin 
behind and sometimes in front of it, but always very near. This 
the eyes sicaddity upon ‘the image while moving the instrument, 
id it gives us a clear impression of the place of the resultant as 
coincident with the point to which the optic axes are converged. 
It is important to observe that while viewing the compound im- 
age in this experiment, we see, on the right of it, the image of 
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