450 ELEMENTARY PHYSIOLOGY less. 



is necessary is that the two spots of the retinas on which 

 the images fall should be similarly disposed towards the 

 centres of their respective yellow spots. Any two points of 

 the two retinas thus similarly disposed towards their re- 

 spective yellow spots (or more exactly to the points in which 

 the visual axes end), are spoken of as corresponding 

 points ; and any two images covering two corresponding 

 areas are conceived of as coming from a single object. It 

 is obvious that the inner (or nasal) side of one retina 

 corresponds to the outer (or cheek) side of the other. 



11. The Judgment of Solidity. — ir/ien, a body of 

 moderate size, ascertained by touch to be solid, is viewed loith 

 both eyes, the images of it, formed by the two eyes, are 

 necessarily different (one shotciny more of its right side, the 

 other of its left side). Nevertheless, they coalesce into a 

 common image, which gives the impression of solidity. 



Conversely, if the two images of the right and left 

 aspects of a solid body be viade to fall upon the retinas of 

 the two eyes in such a way as to coalesce into a common 

 imxtge, they are judged by the mind to proceed from the 

 single solid body xvhich alone, under ordinary circumstances, 

 is competent to produce them. 



The stereoscope is constructed upon this princij^le. 

 Whatever its form, it is so contrived as to throw the images 

 of two pictures of a solid body, such as would be 

 obtained by the right and left eye of a spectator, on to 

 such parts of the retinas of the person who uses the 

 stereoscope as would receive these images, if they really 

 proceeded from one solid body. The mind nnmediately 

 judges them to arise from a single external solid body, and 

 sees such a solid body in place of the two pictures. 



The operation of the mind upon the sensations pi'esented 

 to it by the two eyes is exactly comparable to that which 

 takes place when, on holding a marble between the finger 

 and thumb, we at once declare it to be a single sphere 

 (p. 439). That which is absolutely presented to the mind by 

 the sense of touch in this case is by no means the sensa- 



