NATURE AND BASIS OF FUSION 331 



being utterly dependent upon the stimulation of corresponding points 

 for his singleness of vision, his perception of space and his visual com- 

 fort are at the mercy of any slightest pathological or traumatic disturb- 

 ance of the neuromuscular tie-rod which, at his bidding, turns his team 

 of eyes. 



The Nature and Basis of Fusion — By this time, the reader may have 

 in his mind a rather confused idea as to what 'fusion' actually is. Where 

 the separate monocular images are perfectly identical, as when two prints 

 from the same negative are placed in a stereoscope, the binocular fusion- 

 image differs in no way from the monocular image on either side. The 

 fusion-image could be adequately represented by projecting on a screen, 

 superimposed on each other, the two pictures on such a stereoscope card. 

 But this is a special case — ordinarily, the objects at which we look 

 binocularly have depth or thickness, and our two monocular images of 

 them are not identical. We have noted above that the pattern of the 

 everyday binocular fusion-image is not such that it could be represented 

 by mere superposition of the monocular images. We can perhaps imagine 

 that in some way the whole of a right-eye image is integrated with the 

 whole of a slightly-differing left-eye image, without this resulting in an 

 effect like that of superposition. But is the fusion-image, whether tri- 

 dimensional or flat, of this all-of-right-plus-all-of-left character? 



If it really were, then we should expect to find in binocular vision two 

 phenomena which it does not in fact exhibit : {a) binocular visual acuity 

 should be greater than monocular*; {b) binocular brightness should be 

 greater than monocular. Neither of these things is true of human vision 

 in general, though there does seem to be some summation of the monoc- 

 ular brightnesses in intensities close to the rod threshold. If binocularity 

 in itself conferred higher visual acuity, or increased the overall sensitivity 

 of the visual mechanism to light, then these great advantages would 

 alone be enough to account for the repeated evolution of binocularity 

 by both diurnal and nocturnal vertebrates of all sorts. The parallel visual 

 axes of such forms as the owls, galagos, and deep-sea fishes have indeed 

 been very often explained on the assumption that the binocular-vision 

 phenomena of such animals include a summation of the two monocular 

 brightnesses. And we have seen reason to suspect that binocular acuity 



*Binocular visual acuity in this sense, which is the resolving power of the two eyes together 

 as compared with that of one eye alone, is not to be confused with the more common term 

 'stereoscopic visual acuity'. This latter term refers to the accuracy of binocular distance- or 

 depth-perception. 



