t88i. 11 Trans. N. Y. Ac. Scl. 



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 P C is the apparent position 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 PC 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 again the binocular centre, and the face be lowered and withdrawn till 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 exterml 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 i = interocular distance, R L. 



" a = interval between the corresponding points, A and B. 



" b = distance, G E, between card and observer. 



" X = 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, 



a b 



X = ± 



t ± a 



Applying this formula, Brewster constructed a table ot distances for 

 the bmocular centre. For negative values it is seen that x 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 ft is large, or the lines are viewed very obliquely, as 

 from R" and L". If the 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 



