348 



THE SPECIAL SENSES. 



outlines of one are in blue and the other in red. When looked at, 

 therefore, the picture gives an ordinary flat view with confused 

 red-blue outlines. If, however, one holds a piece of red glass in 

 front of the left eye and a piece of blue glass in front of the right eye, 

 or more conveniently uses the pair of spectacles provided which 

 have blue glass on one side, red on the other, then the picture stands 

 out at once in solid relief with surprising distinctness and as a 

 black and white object only. The red and blue glasses in this case 

 simply serve to throw the right-eyed image on the right eye and the 

 left-eyed image on the left eye. Assuming that the right-eyed 

 image is outlined in red, then the blue glass should be in front of the 

 right eye. This glass will absorb the red rays completely so that 

 the red outlines in the picture will seem black and a distinct right- 

 eyed picture is thrown on the right eye, distinct enough to make us 

 overlook the much fainter image in blue, which is also trans- 

 mitted through the blue glass. The red glass before the left 



eye cuts out, in the same 

 way, the right-eyed image 

 and presents in dark outline 

 the left-eyed image. By 

 simply reversing the spec- 

 tacles the right-eyed image 

 may be thrown upon the 

 left eye and vice versa. 

 Under these conditions the 

 picture for most persons 

 may be seen in inverted 

 relief (pseudoscopic vision), 

 objects in the foreground 

 receding into the background. This inversion of the relief when 

 the projection upon the retinas is reversed is a striking indica- 

 tion of the potency of the normal projection as a factor in our 

 judgments of solid objects. It will be observed, 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. 152, 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. 324, each picture gives a double image, since it falls on 



Fig. 152. Stereoscopic picture of an octahe- 

 dral crystal. May be combined stereoscopically 

 by relaxing the accommodation by the method 

 of heteronymous diplopia. Hold the object at a 

 distance of a foot or more and gaze beyond. 



