IN SINGLE AND BINOCULAR VISION. 357 



in size between the two unequal monocular ones. This perfect coalescence of the 

 two images he considers as demonstrated, and he deduces from it the important 

 conclusion, that, if it were otherwise, " objects would appear single only when the 

 optic axes converge immediately forwards." That is, we see objects single when 

 the optic axes converge laterally in virtue of the coincidence of two unequal 

 images. 



These extraordinary results are obviously subversive of the established laws 

 of vision, but especially of the law of visible direction ; and if they are true, they 

 must arise from a sudden change in the properties of the humours, or in the func- 

 tions of the retina. The lesser image may become greater, or the greater less, by 

 a variation in the refractive density or the form of the cornea and the crystalline 

 lens, or, what would be more probable, the retina may become subject to a new 

 law of visible direction. Assuming this to be the case, we must suppose the 

 change of law to take place in each eye, so that the larger image must be seen 

 less, and the smaller image seen greater, than they really are. Now, this change 

 must take place instantaneously at the moment of coalescence, for the two images 

 retain their proper magnitude till their apparent union takes place ; and the eye 

 must recover its ordinary functions as instantaneously, for the moment we inter- 

 cept one of the images the other resumes its proper size. 



In order to understand what the nature of this supposed change actually is, 

 let MN, M'N' (fig. 3, 4) be the two eyes, AB the larger image, and a b the smaller 

 one ; then if C be the centre of curvature of the retina, the points A,B will be seen 

 in the directions, An, Em intersecting at C, and the points a, b in the directions as, br 

 intersecting at C'. But when these separate images coalesce, in consequence of 

 AB becoming less and b c greater, the points A,B, a, b must be seen in the direc- 

 tions A.n, Bo, av, bt, intersecting at new centres of visible direction c, c', the one 

 farther from, and the other nearer to, the retina. If we now shift the larger picture 

 to the right eye, and the smaller to the left eye, the function of the retina will be 

 again changed : the left eye MN will have its lines of visible direction as in fig. 4, 

 and the right eye as in fig. 3. Such an oscillation of the binocular centre of visible 

 direction on each side of the monocular centre, produced solely during the attempt 

 to unite unequal images, would indicate a function of the retina so extraordinary, 

 that the most incontrovertible experiments, and the universal experience of accu- 

 rate observers, could alone give it credibility. 



There is no doubt that the two unequal images appear to coalesce ; but if we 

 make the outlines of the squares and circles luminous, by pricking small holes in 

 their outlines, and exposing them to very strong light, we shall find it impossible 

 to produce a coincidence. The best way to make this experiment is to take two 

 lines, AB, a , fig. 5, of unequal lengths, and with a large pin to perforate the 

 lines at A,B, a, b, so that when we attempt to unite them, as at fig. 6, we shall 

 see with perfect distinctness then- four luminous extremities. When the point a 



VOL. XV. PART III. 5 D 



