214 RECEXT PROGRESS OF THE THEORY OP VISION. 



really constitutes the chief advantage of the eye over 

 other optical instruments. 



Indeed the peculiar way in which we are accustomed 

 to give our attention to external objects, by turning it 

 only to one thing at a time, and as soon as this has been 

 taken in hastening to another, enables the sense of vision 

 to accomplish as much as is necessary ; and so we have 

 practically the same advantage as if we enjoyed an accu- 

 rate view of the whole field of vision at once. It is not in 

 tact until we begin to examine our sensations closely that 

 we become aware of the imperfections of indirect vision. 

 Whatever we want to see we look at, and see it accurately ; 

 what we do not look at, we do not as a rule care for at 

 the moment, and so do not notice how imperfectly we 

 see it. 



Indeed, it is only after long practice that we are 

 able to turn our attention to an object in the field of 

 indirect vision (as is necessary for some physiological 

 observations) without looking at it, and so bringing it 

 into direct view. And it is just as difficult to fix the 

 eye on an object for the number of seconds required to 

 produce the phenomenon of an after-image.* To get 

 this well defined requires a good deal of practice. 



A great part of the importance of the eye as an organ 

 of expression depends on the same fact ; for the move- 

 ments of the eyeball — its glances — are among the most 

 direct signs of the movement of the attention, of the 

 movements of the mind, of the person who is looking 

 at us. 



Just as quickly as the eye turns upwards, downwards, 

 and from side to side, does the accommodation change, 

 so as to bring the object to which our attention is at 

 the moment directed into focus ; and thus near and dis- 

 tant objects pass in rapid succession into accurate view. 



' Vide infra, p. 254. 



