874 



STEREOSCOPIC VISION. JUDGMENT OF SOLIDITY. 



outward from the plane of the paper toward the observer. From these two 

 fundamental experiments all stereoscopic pairs of pictures may be easily analyzed; 

 particularly, it will be seen also that if in Fig. 305, III, the two pictures be ex- 

 changed, so that R lies in the place of L, the impression of a truncated pyramidal 

 hollow vessel must result. 



_ Two stereoscopic pictures, which are so constructed that one contains an 

 object taken from in front and above, the other the object taken from in front 

 and below (for example, if the figures in Fig. 305, III, had the lines A B and 

 a'b as base line), are never united by means of the stereoscope. If these two 

 figures be turned so that they have an oblique position (so that the corners C t c 



II 



D b 



FIG. 305. I, Diagrammatic Representation of Brewster's stereoscope; II, that of Wheatstone; III, two stereo- 

 scopic drawings; IV, v. Helmholtz's telestereoscope. 



deviate toward the right and downward) , the impression of solidity will be more 

 and more interfered with; but Landois was able to preserve the stereoscopic 

 appearance up to a similar rotation of 30. 



The process of stereoscopic vision has been explained also in another 

 way In the figure R and L (Fig. 305, III) only A B C D and a b c d fall 

 on identical points of the retinas, and, therefore, these alpne can coin- 

 cide (or in another convergence of the visual axes only E F G H and 

 f g h coincide, for the same reason). If it be supposed that the quad- 

 bases of the figures coincided first, it has been further assumed 

 that both eyes then make a quick "groping" motion toward the apex 



