72 2 TEXT-BOOK OF PHYSIOLOGY 



sensation. If the direction of either visual axis be changed by pressure on 

 the eyeball, there arise two sensations, and the object appears to be doubled. 

 The reason assigned for this, in the first instance, is that the two images 

 fall into the foveae, on two corresponding points; while in the second instance 

 they fall on non-corresponding points. It would appear, therefore, that for 

 the' purpose of seeing an object singly when the eyes are directed toward 

 it, the rays emanating from it must fall on corresponding parts of the retina. 



As all portions of the retina are sensitive 

 to light, though in varying degrees, it is 

 not essential that the images always fall 

 in the foveae. The parts of the retinae 

 which correspond physiologically are 

 shown hi Fig. 311. In this figure the 



retinal area is divided into quadrants 

 Fro. 7ii. CORRESPONDING AREAS OF THE , ^i j i_ * i t * 



R ETINA . by vertical and horizontal lines of sepa- 



ration, as they are termed. If one retina 



is placed in front of or over the other, it will be found that the quadrants 

 bearing similar letters cover each other. So long as the rays of light, 

 entering the eye, fall on corresponding areas the sensation of but one 

 object arises. If, however, they fall on non-corresponding areas, two sen- 

 sations arise. Normal binocular vision enlarges very considerably the area 

 of the visual field, permits of a better estimation of the size and distance 

 of objects, enables the mind to form more readily a perception of depth, 

 and increases the intensity of sensations. 



The Horopter. When the eyes are in a so-called secondary position 

 that is, in a position in which the visual axes are converged and directed 

 to a point in front of and in the middle plane of the body it will be found 

 on examination that rays of light from a number of other objects enter the 

 eye, pass through the nodal point, and fall on corresponding parts of the 

 two retinae and give rise to but single images. All such points lie, for the 

 horizontal line of separation, on a line termed the horopter. The form of 

 this line is that of a circle which passes through the fixation point and the 

 two nodal points. Any object on the horopter will give rise to but a single 

 image. This is shown in Fig. 312, in which the objects I, II, III project 

 their rays into both eyes and upon corresponding areas. 



In addition to the horopter for the horizontal line of separation, there 

 is also an horopter for the vertical line of separation. At a distance of two 

 meters the vertical horopter is a plane. Within this distance it is concave to 

 the face; beyond this distance it is convex. 



An object which lies either in front of or behind the fixation point will 

 project its rays on parts of the retinae which do not correspond, and hence 

 give rise to double images. This is evident from examination of Fig. 313. 

 While the eyes are directed to figure 2, of which there is but a single image, 

 the objects B and A give rise to double images, for reasons already 

 given. If the eyes are now directed to B, double images will be formed of 

 2 and A. 



^ At all times, therefore, double images are formed on the retinae the 

 existence of which is scarcely noticed unless the attention is directed to them. 

 This is due to the fact that many of the images fall on the peripheral, less 

 sensitive parts of the retinae. At the same time, from a want of accommo- 



