358 



THE SPECIAL SENSES. 



eye cuts out, in the same way, the right-eyed image and presents 

 in dark outline the left-eyed image. By simply reversing the 

 spectacles the right-eyed image may be thrown upon the left eye 

 and vice versa. Under these conditions the picture for most per- 

 sons may be seen in inverted 

 relief (pseudoscopic vision), 

 objects in the foreground re- 

 ceding into the background. 

 This inversion of the relief 

 when the projection upon the 

 retinas is reversed is a strik- 

 ing indication of the potency 

 of the normal projection as a 



Fig. 155. Stereoscopic picture of an octahe- /. -, r 



dral crystal. May be combined stereoscopically lactor in OUr judgments OI 



by relaxing the accommodation by the method _i; i i 4. TJ. '11 1 ^1 



of heteronymous diplopia. Hold the object at a Solid objects. It Will be ob- 



distance of a foot or more and gaze beyond. served, moreover, that those 



pictures that show least 



mathematical perspective are the most readily inverted, and that 

 the ability to invert the picture varies in different individuals; in 

 some, what we have called the binocular perspective, founded upon 

 the dissimilar images, prevails over the mathematical perspective 

 more readily than in others. 



Stereoscopic pictures may also be combined very successfully 

 without the use of a stereoscope by virtue of the phenomenon of 

 physiological diplopia. If, for instance, two stereoscopic drawings, 

 such as are represented in Fig. 155, are held before the eyes and one 

 relaxes his accommodation so as to look through the pictures, as it 

 were, to a point beyond, then, in accordance with what was stated 

 on p. 352, each picture gives a double image, since it falls on 

 non-corresponding parts of the two retinas. Four pictures, there- 

 fore will be seen, all out of focus. With a little practice one can so 

 converge his eyes as to make the two middle images come together, 

 and since one of these is an image of the right-eyed picture and is 

 falling on the right eye, and the other is a left-eyed picture falling 

 on the left eye, the combination of the two fulfills the necessary 

 conditions for binocular perspective. The figure stands out in 

 bold relief. 



Explanation of Binocular Perspective. Our perception of 

 solidity or relief is a secondary psychical act, and, so far as the binoc- 

 ular element is concerned, it is based upon the fact that the images 

 are slightly different on the two retinas; but why this dissimilarity 

 should produce an inference of this kind is not entirely understood. 

 Certain facts have been pointed out as having a probable bearing 

 upon the mental process. In the first place, in stereoscopic pictures, 



