THE SENSE OF SIGHT. 



629 



FIG. 303. RETINA OF 

 A RABBIT. OPTO- 

 GRAM OF A WIN- 

 DOW FOUR METERS 

 DISTANT, a. Yellow 

 spot, b, b. White 

 streak of nerve - 

 fibers. (Kuhne.) 



instance, is that the two images fall into the foveae, 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 eman- 

 ating 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 in Fig. 304. In this figure the 

 retinal area is divided into quadrants by 

 vertical and horizontal lines of separation, 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 sensations arise, clue to the stimulating 

 action of two images. Normal binocular 

 vision enlarges very considerably the area of 

 the visual field, permits of a better estimation of the size and dis- 

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

 of depth, increases the intensity of sensations and makes sensation 

 more uniform by offsetting retinal rivalry. 



The Horopter. When the eyes are in the so called secondary 

 position, that is, in a position in which the visual axes are con- 

 verged 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 hori- 

 zontal 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. 305, in which the objects I, II, III project 

 their rays into both eyes which fall on 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 



FIG. 304. CORRESPONDING AREAS OF THE 

 RETINA. 



