VISUAL PERCEPTIONS. 777 



conscious of a number of sensations of bright and dim lights, of green, 

 brown, black, etc., but these sensations are so related to each other and by 

 virtue of cerebral processes so fashioned into a whole, that we " see a tree." 

 We sometimes, in illustration of such an effect, speak of an image or pic- 

 ture 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 contempora- 

 neous 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 sensa- 

 tions ; 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 sepa- 

 rately and for the two eyes combined. 



672. 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, localize 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 sensations ; and the relative positions, together with the re- 

 lative intensities and qualities (color) of the sensations arising from any 

 object determine our perception of the object. It need hardly be remarked 

 that this localization is purely subjective. We simply determine the posi- 

 tion of the sensation in the field of vision (which is itself a wholly subjective 

 matter) ; we do not determine the position of the object. The connection 

 between the position of the object in the external world and the position of 

 the sensation in the field of vision cannot be determined by visual observa- 

 tion alone. All the information which can be gained by the eye is limited 

 to the field of vision, and provided that the relative position of the sensa- 

 tions in the field of vision remained the same, the actual position of exter- 

 nal objects might, as far as vision is concerned, be changed without our being 

 aware of it. 



As a matter of fact the field of vision in one important particular does 

 not correspond to the field of external objects. The image on the retina is 

 inverted ; the rays of light proceeding from an object which by touch we 

 know to be on what we call our right hand, fall on the left-hand side of the 

 retina. If, therefore, the field of vision correspond to the retinal image, 

 the object would be seen on the left hand. We, however, see it on the right 

 hand, because we invariably associate right-hand tactile localization with 

 left-hand visual localization ; that is to say, our field of vision, when inter- 

 preted by touch, is a re-inversion of the retinal image. 



The dimensions of the field of vision of a single eye are about 145 degrees 

 for the horizontal and 100 degrees for the vertical meridian, the former 

 being distinctly greater than the latter. The horizontal dimension of the 

 field of vision for the two eyes is about 180 degrees. By movements of the 

 eyes, however, even apart from those of the head, the extent may be con- 

 siderably increased. 



The satisfactory perception of external objects requires distinct vision ; 

 and of this, as we have already said, the formation of a distinct image on 

 the retina is an essential condition. We can receive visual sensations of all 

 kinds with the most imperfect dioptric apparatus, but our perception of an 

 object is precise in proportion to the clearness of the image on the retina. 



673. Region of distinct vision. If we take two points, such as two black 



