658 THE SENSE OF SIGHT. 



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

 corresponding to this motion, as in reading, 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 it& 

 activity more or less exclusively upon the senses of sight, 

 hearing, 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 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 mind to perception 

 when otherwise engaged. The attention which is thu& 

 necessary for vision, is necessary also to analyse 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 another. The sensation becomes more intense 

 according as the particular object is at the time the 

 principal object of mental contemplation. 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. 1 86, we may in succes- 

 sion have a vivid perception of the whole, 

 or of distinct parts only ; of the six triangles 

 near the 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 attention. Hence it is 

 that architectural ornaments have an enlivening effect on 



