BINOCULAR VISION 



693 



the line varies somewhat from a regular curve, and also varies in 

 different meridians, this is due to differences in refraction, etc., and 

 the principle is not altered. 



If the visual areas of the two retinae are superimposed, the fixation- 

 points coinciding, it becomes evident that a portion only of the two fields 

 can have corresponding points. This is the light portion shown in Fig. 

 178, which may be called the binocular field of vision. Binocular vision 

 must be impossible in the temporal portion of each visual area. 



It is undoubtedly true that education and habit have much to do with 

 the correction of visual impressions and the just appreciation of the size, 

 form and distance of objects. In the remarkable case of Kaspar Hauser, 

 who is said to have been kept in total darkness and seclusion from the 

 age of five months until he was nearly seventeen years old, the appre- 

 ciation of size, form and distance- were acquired by correcting and 

 supplementing the sense of 

 sight by experience. This 

 boy at first had no idea of 

 the form of objects or of 

 distance, until he had learned 

 by touch, by walking etc., 

 that certain objects were 

 round and others were square, 

 and had actually traversed the 

 distance from one object to 

 another. At first all objects 



Fig. 178. Binocular field of vision (Forster). 

 F, fixation-point; B, B, blind spots. 



appeared as if painted upon 

 a screen. Such points as 

 these it would be impossible to observe accurately in infants ; but young 

 children often grasp at distant objects, apparently under the impression 

 that they are within reach. It must be admitted, however, that the 

 account of the case of Kaspar Hauser is rather indefinite ; but it is 

 certain that even in the adult, education and habit greatly improve the 

 faculty of estimating distances. 



Careful observations leave no doubt of the fact that monocular vision 

 is incomplete and inaccurate, and that it is only when two images are 

 formed, one upon either retina, that vision is perfect. The sum of 

 actual knowledge on this important point is expressed in the follow- 

 ing quotation from Giraud-Teulon : 



" Monocular vision only indicates to us immediately, visual direction, 

 and not precise locality. At whatever distance a luminous point may be 

 situated in the line of direction, it forms its image upon the same point 

 in the retina. 



