790 



THEORY OF STEREOSCOPIC VISION. 



Two stereoscopic pictures, which are so constructed that the one contains the body from the 

 front and ateve.anS the other it from the front and below (suppose in fig 569 the lines A B, 

 and ! a "wire the ground lines), can never be superposed by means of the stereoscope. 



This process has been explained in another way. Of the two figures, R and L 

 (fie 569) only A B C D, and a b c d y fall upon identical points of the retina, hence 

 these alone can be superposed ; or, when there is a different convergence of the 

 visual axis, only E F G H, and ef g k, can be superposed for the same reason. 

 Suppose the square ground surfaces of the figures are first superposed in order to 

 explain the stereoscopic impression, it is further assumed that both eyes, after 

 superposition of the ground squares, are rapidly moved towards the apex of the 



r 



B 



\ / 



Fig. 570. Fig. 571. 



. Wheatstone's stereoscope. Scheme of Brewster's stereoscope. 



pyramid. As the axis of the eyes must thereby converge more and more, the apex 

 of the pyramid appears to project ; as all points which require the convergence of 

 the eyes for their vision appear to us to be nearer (see below). Thus, all corre- 

 sponding parts of both figures would be brought, one after the other, upon identical 

 points of the retina by the movements of the eyes (Briicke). 



It has been urged against this view that, the duration of an electrical spark 

 suffices for stereoscopic vision (Dove) a time which is quite insufficient for the 

 movements of the eyes. Although this may be true for many figures, yet in the 

 correct combination of complex or extraordinary figures, these movements of the 

 visual axes are not excluded, and in many individuals they are distinctly advan- 

 tageous. Not only the actual movements necessary for this act, but the sensa- 

 tions derived from the muscles are also concerned. 



When two figures are momentarily combined to form a stereoscopic picture, there 

 being no movement of the eyes, clearly many points in the stereoscopic pictures are 

 superposed which, strictly speaking, do not fall upon identical points of the retina. 

 Hence we cannot characterise the identical points of the retina as coinciding 

 mathematically ; but from a physiological point of view we must regard such points 

 as identical, which, as a rule, by simultaneous stimulation, give rise to a single 

 image. The mind obviously plays a part in this combination of images. There 

 is a certain psychical tendency to fuse the double images on the retinae into one 

 image, in accordance with the fact that we, from experience, recognise the 

 existence of a single object. If the differences between two stereoscopic pictures 



