CHAP, ii.] SIGHT. 535 



sensations; and the relative positions, together with the relative 

 intensities and qualities (colour) of the sensations arising from any 

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

 remarked that this localisation is purely subjective. We simply 

 determine the position 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 observation 

 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 sensations in the field of vision remained the same, the 

 actual position of external objects might, as far as vision. is con- 

 cerned, be changed without our being aware of it. 



As a matter of fact the field of vision in one important par- 

 ticular 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 corresponded 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 localisation with 

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

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



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

 145 for the horizontal and 100 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. 

 By movements of the eyes, however, even apart from those of the 

 head, the extent may be considerably 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. 



Region of Distinct Vision. If we take two points, such as 

 two black dots, only just so far apart that they can be seen dis- 

 tinctly as two when placed near the axis of vision, and then, 

 keeping the axis fixed, move the two points out into the circum- 

 ferential parts of the field of vision, it will be found that the two 

 soon appear as one. The two sensations become fused, as they 

 would do if brought nearer to each other in the centre of the field. 

 The farther away from the centre of the field, the farther apart 

 must two points be in order that they may be seen as two. In 

 other words, vision is much more distinct in the centre of the field 

 than towards the circumference. Practically the region of distinct 



