THE STEREOSCOPE AND VISUAL INTERPRETATION 1 77 



for each such point. Every retinal point furnishes in the movement 

 which corresponds to it a cue for the location of its objective stimulus. 

 Dove 1 made experiments with the stereoscope and found that it was 

 possible to get stereographic effects by a flash-light (one ten-millionth 

 of a second duration). This demonstration which was repeated by 

 several, established for some time the nativistic doctrine and its last 

 great exponent was Hering. According to his theory each light ray 

 gives a light sensation, whose character is determined by the objective 

 stimulus; and a space feeling conditioned by the retinal point stimulated. 

 He does not use eye-movements in spacial vision. Each retinal point 

 has its own space value which is a function of the distance and direction 

 of that point from the fovea. 



From Wundt 2 down every psychologist has dealt with the principles 

 of vision. And several have gone into very great detail. 3 The most 

 readable and interesting book on the subject is by Le Conte. 4 Wundt 

 made an interesting experiment in monocular and binocular vision and 

 his conclusions have gone far toward establishing the present point of 

 view. His purpose was to ascertain the influence of accommodation 

 and convergence on the perception of depth. His instrument consisted 

 of a long black box with observation tubes at one end and a white back- 

 ground at the other. Two parallel slits were placed one inch apart in 

 the top of the box continuing from one of its ends to the other. From 

 these slits two small weights were suspended by black silk threads 

 reaching almost to the bottom of the box. These threads could be 

 moved backward and forward in the slit. It was found that by looking 

 with the two eyes while one of the threads was moved the sensitivity 

 was 4.5 as great as when but one eye was used. Quite a number of 

 experiments along this same line have subsequently been made, but 

 will not be mentioned here on account of the limited space allotted to 

 this discussion. The above account was given to show that this was 

 by no means a neglected field. 



The purpose of this article is to show by means of a stereoscopic 

 demonstration the factors involved in vision. These experiments out- 



1 Dove, Berichte der Berliner Akademie (1841). a Wundt, Beilrage. 



3 Helmholtz, Physiologisehc Optik. * Le Conte, Sight. 



