1881. rt Trans. IN. V. Ac. Scz. 
R and L, be placed above this. If they be directed to the point C, 
this appears in its true position. If the right eye be directed to B and 
the left to A, the axes meet at P; this point Brewster calls the binocu- 
lar centre; and since the retinal images of B and A correspond, the 
visual effect is that of the union of these two external points at the 
binocular centre. Sweeping the glance toward C, the two lines appear 
united in the air, and PC is the apparent pesition of the combination, 
intermediate in direction between two monocular images, which may be 
disregarded or hidden from view with screens. If the convergence 
of visual lines be now diminished, the binocular image is lost until the 
right eye becomes directed to A and the left to B. The two points 
appear united at P’, and the line P’'C now appears in the air on the 
further side of the surface. If the convergence be increased till P is 
again the binocular centre, and the face be lowered and withdrawn till 
the eyes are at R" and L’, then C P” becomes the position of the 
variable external image. And if lowered until R"L” coincides with the 
surface, C P” vanishes at the moment of becoming coincident with the 
prolongation of G C, the median of the triangle A C B. 
Brewster’s formula for determining the distance of the binocular 
centre from G is easily deduced and applied. 
Let z = interocular distance, R L. 
‘« q = interval between the corresponding points, A and B. 
«« 6 = distance, G E, between card and ebserver. 
“ 4 — distance G P, or G P’, which is positive when measured 
toward the observer, negative in the direction opposite. Then, observ- 
ing the usual rule of signs, we have, by Geometry, 
ab 
t+a 
— 
Applying this formula, Brewster constructed a table of distances for 
the binocular centre. For negative values it is seen that + becomes 
infinite when the visual lines become parallel ; and, if they be slightly 
divergent, the binocular centre is far in the rear of the observer. 
Either of these conditions would make binocular vision impossible if 
the theory be correct. In testing the experiment with trained eyes, it 
is found quite possible to secure binocular fusion of the images of A 
and B when the interval between these points equals or slightly ex- 
ceeds the interocular distance. It is also found that fusion of the 
images of the whole line at any given instant is impossible, especially 
when the angle § is large, or the lines are viewed very obliquely, as 
from R" and L’. Ifthe images of A and B fall on corresponding 
retinal points, the resulting sensation is binocular fusion, whether the 
visual lines be convergent, parallel or divergent ; and the images of any 
