Sir D. Brewster on the Law of Position in Vision. 439 



peculiarities of this preparation ; and they furnish us with ad- 

 ditional evidence of the remarkable changes induced in bodies 

 by the solar emanations, which we were comparatively igno- 

 rant of previously to the publication of the Daguerreotype and 

 photographic processes. 



I remain, dear Sir, 



Yours faithfully, 

 Falmouth, May 8, 1844. Robert Hunt. 



LXIV. On the Law of Visible Position in Single and Bi- 

 nocular Vision, and on the representation of Solid Figures 

 by the union of dissimilar Plane Pictures on the Retina. By 

 Sir David Brewster, K.H.,D.C.L., F.R.S., and V.P.R.S. 

 Edin. 



[Continued from p. 365 and concluded.] 



4. On the Binocular Vision of Figures of Different Magnitudes. 

 iyt R. WHEATSTONE seems to have been the first person 

 ■*"-*■ who made experiments on the binocular vision of un- 

 equal figures. Having drawn on separate pieces of paper "two 

 squares or circles, differing obviously, but not extravagantly, in 

 size," he placed them in the stereoscope, and concluded from 

 his observations that the two unequal •pictures "coalesced, and 

 occasioned asingleresultant perception;" and that thebinocular 

 image thus perceived was apparently intermediate in size be- 

 tween the two unequal monocular ones. This perfect coa- 

 lescence 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 functions 

 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 



