714 SPECIAL SENSES. 



distance, this can not occur. This illustrates the fact that there are corre- 

 sponding points in a large part of the sensitive layer of the retina, as well as 

 in the fovea centralis. By these experiments, the following facts have been 

 ascertained : With both eyes fixed upon an object, another object moved to 

 one side or the other can be distinctly seen only when it is carried in a cer- 

 tain curved line. On either side of this line, the object appears double. 

 This line, or area for the line may have any direction is called the horop- 

 ter. It was supposed at one time to be a regular curve, or a portion of a circle 

 drawn through the fixed point and the points of intersection of the rays of 

 light in each eye. Although it has been ascertained that 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 retinas be 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. 258, which may be called 

 the binocular field of vision. 

 Binocular vision must be im- 

 possible in the temporal por- 

 tion of each visual area (Net- 

 tleship). 



It is undoubtedly true that 

 education and habit have a 

 great deal to do with the cor- 



FlG. 258. Binocular Held of vision (Nettleship, after Forster.) roofinn rf vianol irnnvoecirma 

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



and the just appreciation of 



the size, form and distance of objects. In the remarkable case of Casper 

 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 appreciation 

 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 appeared as if 

 painted upon a screen. Such points as these it would be impossible to accu- 

 rately observe in infants ; but young children often grasp at remote objects, 

 apparently under the impression that they were within reach. It must be 

 admitted, however, that the account of the case of Casper 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 absolutely perfect. The sum of actual 

 knowledge upon this important point is expressed in the following quotation 

 from Giraud-Teulon : 





