662 



PHYSIOLOGY 



position. In this judgment or interpretation a number of separate 

 processes must be involved. The fact that the eyes are not accom- 

 modated for the thread evokes at once the associated movements 



L.. 



FIG. 303. The eyes are directed to the point l>. A thread hung obliquely at a 

 under these circumstances gives rise to the images shown in the upper figures 

 i.e. two images which do not lie on corresponding points. Nevertheless the 

 thread is seen as single. 



which would be necessary, if time allowed, to bring the non-corre- 

 sponding images on to corresponding points of the retina. We do not 

 therefore assume a doubleness of an object, even when its images 

 fall on non- corresponding points, unless our gaze is voluntarily directed 

 on the object. 



THE JUDGMENT OF SOLIDITY. The fusion of non-corre- 

 sponding images to a single visual conception is responsible for our 



appreciation of solidity. If we look at 

 any object which is not too far from the 

 eyes, first with the right and then with the 

 left eye, we shall see that the images in 

 the two eyes are not identical, If we 

 look, for instance, at a truncated pyramid, 

 we shall see with the right eye rather more of the right side of 

 the pyramid and with the left eye rather more of the left side 

 (Fig. 304). If these two images a and b be so arranged that they 

 fall on corresponding points of the two retinae, there is no confusion 

 of sensation, but the resulting impression is that of a solid object. 

 This is the principle involved in the stereoscope, which allows us 

 to combine two images of this character so that they fall on corre- 

 sponding points of the two retinas. 



