190 RECENT PROGRESS OF THE THEORY OF VISION. 



this rapidity of movement which 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 accurate view of the whole field of vision 

 at once. It is not in fact until we begin to examine our sen- 

 sations 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. 1 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 movements 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 distant objects pass in rapid suc- 

 cession into accurate view. 



All these changes of direction and of accommodation take 

 place far more slowly in artificial instruments. A photographic 

 camera can never show near and distant objects clearly at once, 

 nor can the eye ; but the eye shows them so rapidly one after 



i Vide infra, p. 224. 



