HEARING. 1 1 5 



is not always the same. Sometimes the two colours are 

 blended as completely as if both affected one retina : at 

 others there is a contest between the two, first one, then 

 the other predominating in the blending. 2. Judgment of 

 distance. We judge of the distance of any object, chiefly 

 by the degree of convergence of the visual axes, which we 

 find necessary in order to fix both eyes upon it. Conse- 

 quently, if one eye is shut and two or more objects of the 

 same form but of different sizes are placed before the other 

 at such distances that their retinal images cover equal 

 areas, their respective distances cannot be distinguished. 

 If both eyes are used a correct judgment can be formed 

 without difficulty. 3. Judgment of solidity. By explor- 

 ing an object with the eyes, i.e., by fixing them successively 

 on different points of its visible surface, we are able to 

 judge of the relative distances of these points, as well as 

 of their directions. It can, however, be shown that this 

 process is not ordinarily employed in judging of the form 

 and solidity of objects, but that the mind accomplishes this 

 instantaneously, by the blending of the two dissimilar 

 images which are received by the two retinae, whenever 

 both eyes are fixed on some point in a solid object at a 

 short distance from them. The point so contemplated is 

 of course seen single, others are for the most part seen 

 double : notwithstanding this, we are not conscious of any 

 confusion of images. 



Hearing. 



The process of hearing consists (i) in the production of 

 vibratory movements of the membrana tympani, which are 

 synchronous with the sound-vibrations of air in the 

 meatus ; (2) in the communication of these vibrations to 

 the liquid contained in the labyrinth ; and (3) in the pro- 

 duction of vibrations in all those parts of the lamina 



I 2 



