SEC. 3. VISUAL PERCEPTIONS. 



Hitherto we have studied sensations only, and have considered 

 an external object, such as a tree, as simply a source of so many 

 distinct sensations, differing from each other in intensity and kind 

 (colour). In the mind these sensations are coordinated into a per- 

 ception. We are not only conscious of a number of sensations of 

 bright and dim lights, of green, brown, black, &c., but these sen- 

 sations are so related to each other and by virtue of cerebral pro- 

 cesses so fashioned into a whole, that we 'see a tree/ We some- 

 times, in illustration of such an effect, speak of an image or picture 

 in the mind corresponding to the physical image on the retina. 



When we look upon the external world, a variety of images 

 are formed at the same time on the retina, and give rise to a 

 number of contemporaneous visual sensations. The sum of these 

 sensations constitutes 'the field of vision,' which varies of course 

 with every movement of the eye. This field of vision, being in 

 reality an aggregate of sensations, is of course a subjective matter; 

 but we are in the habit of using the same phrase to denote the 

 sum of external objects which give rise to the aggregate of visual 

 sensations; in common language the field of vision is 'all that we 

 can see' in any position of the eye, and we have a field of vision 

 for each eye separately and for the two eyes combined. 



Using for the present the words in their subjective sense, we 

 may remark, that we are able to assign to each constituent 

 sensation its place among the aggregate of sensations constituting 

 the field of vision ; we can, as we say, localise the sensation. We 

 can say whether it belongs to (what we regard as) the right-hand 

 or left-hand, the upper or the lower part, of the field of vision. We 

 are able to distinguish the relative positions of any two distinct 



