856 



THE EYE AND VISION 



[CH. LVIII. 



jections of the object being presented simultaneously to the mind by 

 the two eyes. 



Thus, if a cube is held at a moderate distance before the eyes, 

 and viewed with each eye successively while the head is kept 

 perfectly steady, A (fig. 543) will be the picture presented to the 

 right eye, and B that seen by the left eye. Wheatstone has shown 

 that on this circumstance depends in a great measure our conviction 

 of the solidity of an object, or of its projection in relief. If different 

 perspective drawings of a solid body, one representing the image 

 seen by the right eye, the other that seen by the left (for example, 

 the drawing of a cube, A, B, fig. 543), be presented to corresponding 

 parts of the two retinae, as may be readily done by means of the 

 stereoscope, the mind will perceive not merely a single representa- 



\ 



FIG. 543. Diagrams to illustrate how a judgment of a figure of three dimensions is obtained. 



tion of the object, but a body projecting in relief, the exact counter- 

 part of that from which the drawings were made. 



By transposing two stereoscopic pictures a reverse effect is pro- 

 duced; the elevated parts appear to be depressed, and vice versd. 

 An instrument contrived with this purpose is termed a pseudoscope. 

 Viewed with this instrument a bust appears as a hollow mask, and 

 as may readily be imagined the effect is most bewildering. 



The clearness with which the details of an object are perceived 

 irrespective of accommodation, would appear to depend largely on 

 the number of rods and cones which its retinal image covers. Hence 

 the nearer an object is to the eye (within moderate limits) the more 

 clearly are all its details seen. Further, if we want carefully to 

 examine any object, we always direct the eyes straight to it, so that 

 its image shall fall on the two yellow spots, where an image of a 

 given area will cover a larger number of cones than anywhere else 

 in the retina. ^ Moreover, as previously pointed out, each cone in the 

 macula lutea is connected to a separate chain of neurons. 



The importance of binocular vision is very great. If an object is 

 looked at with one eye only, it is impossible to estimate its distance 

 by the sense of vision alone. For instance, if one eye is closed 

 and the other looks at a wire or bar, it is impossible to tell 



