516 THE SENSE OF SIGHT. 



depends wholly on the outline of the part of the retina affected, 

 the sensation alone is adequate to the distinction of only su- 

 perficial forms of each other, as of a square from a circle. But 

 the idea of a solid body, as a sphere, or a body of three or 

 more dimensions, e. g., a cube, can only be attained by the ac- 

 tion of the mind constructing it from the different superficial 

 images seen in different positions of the eye with regard to the 

 object; and, as shown by Mr. Wheatstone and illustrated in 

 the stereoscope, from two different perspective projections of 

 the body being presented simultaneously to the mind by the 

 two eyes. Hence, when, in adult age, sight is suddenly re- 

 stored to persons blind from infancy, all objects in the field of 

 vision appear at first as if painted flat on one surface ; and no 

 idea of solidity is formed until after long exercise of the sense 

 of vision combined with that of touch. 



We judge of the motion of an object, partly from the motion 

 of its image over the surface of the retina, and partly from 

 the motion of our eyes following it. If the image upon the 

 retina moves while our eyes and our body are at rest, we con- 

 clude that the object is changing its relative position with re- 

 gard to ourselves. In such a case the movement of the object 

 may be apparent only, as when we are standing upon a body 

 which is in motion, such as a ship. If, on the other hand, the 

 image- does not move with regard to the retina, but remains 

 fixed upon the same spot of that membrane, while our eyes 

 follow the moving body, we judge of the motion of the object 

 by the sensation of the muscles in action to move the eye. If 

 the image moves over the surface of the retina while the mus- 

 cles of the eye are acting at the same time in a manner cor- 

 responding to this motion, as in reading, we infer that the ob- 

 ject is stationary, and we know that we are merely altering 

 the relations of our eyes to the object. Sometimes the object 

 appears to move when both object and eye are fixed, as in 

 vertigo. 



The mind can, by the faculty of attention, concentrate its ac- 

 tivity more or less exclusively upon the senses of sight, hear- 

 ing, and touch alternately. When exclusively occupied with 

 the action of one sense, it is scarcely conscious of the sensations 

 of the others. The mind, when deeply immersed in contem- 

 plations of another nature, is indifferent to the actions of the 

 sense of sight, as of every other sense. We often, when deep 

 in thought, have our eyes open and fixed, but see nothing, be- 

 cause of the stimulus of ordinary light being unable to excite 

 the mind to perception when otherwise engaged. The atten- 

 tion which is thus necessary for vision, is necessary also to 



