558 



VISION 



viduals with normal acuteness of vision could perceive a displacement of about 

 twice this amount. 



The above-mentioned experiment represents stereoscopic vision in its sim- 

 plest form. When we look at a solid object, not too far removed, first with 

 the right eye and then with the left, the picture we get of the object is not 

 exactly the same for the two eyes. The right eye sees a little more of the 

 right side of the object, the left a little more of the left side. In Fig. 250 

 are represented the images of a truncated pyramid as seen by the two eyes 



separately. A glance at the two is suf- 

 ficient to convince one that these two 

 images could not possibly fall on cor- 

 responding points of the retina?. But 

 if these drawings be held before the 

 eyes so that the one may be seen with 

 the right eye and the other with the left, 

 the two fuse together in our minds 

 into a picture of an actual pyramid. 



Unless one is accustomed to accom- 

 modate the eyes without converging the 

 lines of vision this experiment is diffi- 

 cult. The same result is obtained with- 

 out accommodation if the two pictures 

 be refracted separately into the two eyes 

 by means of lenses. This is the princi- 

 ple of the common stereoscope of which 

 Brewster's form (Fig. 251) may be taken 



as an example. It is apparent from the figure that two pictures situated at 



I and r will be refracted so as to be superimposed at o. 



For stereoscopic vision to be of any importance the object must not be too 

 far removed, for then the images belonging to the two eyes would not be 

 noticeably different. The ordinary stereoscopic views of landscapes are photo- 

 graphed with a double camera so that the plates are farther apart than the 

 distance between the two eyes. Consequently such photographs combined 

 give us an impression of solidity such as natural vision does not afford. 



EEFERENCES. Aubert, " Physiologische Optik" (Graefe-Saemisch's " Hand- 

 buch der Augenheilkunde," ii, 2, Leipzic, 1876). FicJc, Kuhne and Bering, 

 " Gesichtssinn " (Hermann's Handbuch der Physiologie, iii, 1, Leipzic, 1879). 

 Helmholtz, " Handbuch der physiologischen Optik," second edition, Hamburg 

 and Leipzic, 1886-1896. The last named cites a very complete literature of the 

 subject of physiological optics. Hering, " Zur Lehre vom Lichtsinn," Wien, 

 1878. v. Kries, several articles on the " Physiologie der Gesichtsempfindungen," 

 Leipzic, 1897-1902. v. Kries, Nagel and Schenck, "Gesichtssinn ' (Handbuch 

 der Physiologie, iii, 1, Braunschweig, 1904). Wundt, "Text-book of Physio- 

 logical Psychology," translated by E. B. Titchner, New York, 1905. 



FIG. 251. Brewster's stereoscope. 



