THE SENSES. 223 



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 conclude that 

 the object is changing its relative position with regard 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 fol- 

 low the moving body, we judge of the motion of the object by the sen- 

 sation of the muscles in action to move the eye. If the image moves 

 over the surface of the retina while the muscles of the eye are acting 

 at the same time in a manner corresponding to this motion, as in read- 

 ing, we infer that the object 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 activity more 

 or less exclusively upon the sense of sight, hearing, and touch alternately. 

 When exclusively occupied with the action of one sense, it is scarcely con- 

 scious of the sensations of the others. The mind, when deeply immersed 

 in contemplations 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, because of the stimulus 

 of ordinary light being unable to excite the brain to perception, when 

 otherwise engaged. The attention which is thus necessary for vision, is 

 necessary also to analyze what the field of vision presents. The mind does 

 not perceive all the objects presented by the field of vision at the same 

 time with equal acuteness, but directs itself first to one and then to an- 

 other. The sensation becomes more intense, according as the particular 

 object is at the time the principal object of mental contem- 

 plation. Any compound mathematical figure produces a 

 different impression according as the attention is directed 

 exclusively to one or the other part of it. Thus in Fig. 

 383, we may in succession have a vivid perception of the 

 whole, or of distinct parts only; of the six triangles near the FIG. 383. 

 outer circle, of the hexagon in the middle, or of the three 

 large triangles. The more numerous and varied the parts of which a 

 figure is composed, the more scope does it afford for the play of the atten- 

 tion. Hence it is that architectural ornaments have an enlivening effect 

 on the sense of vision, since they afford constantly fresh subject for the 

 action of the mind. 



Color Sensations. If a ray of sunlight be allowed to pass through 

 a prism, it is decomposed by its passage into rays of different colors, which 

 are called the colors of the spectrum; they are red, orange, yellow, green, 



