872 SINGLE VISION. IDENTICAL RETINAL POINTS. 



2. In the secondary position of the eyes, with convergent visual axes, the 

 horopter for the transverse lines of division is a circle passing through the nodal 

 points of both eyes (Fig. 304, KKj) and through the point fixed (I, II, III). The 

 horopter of the vertical lines of division is in this position perpendicular to the 

 plane of fixation. 



3. In the (symmetrical) tertiary positions, in which horizontal and vertical 

 lines of division form angles, the horopter for the vertical lines of division is a 

 straight line inclined toward the horizon. For identical points of the horizontal 

 lines of division there is no horopter for these positions, as the rays from these 

 points do not meet at a distance. 



4. In the unsymmetrical tertiary positions (with rotation), in which the 

 point fixed is at an unequal distance from the two nodal points, the horopter 

 is a curve of complicated form. 



It will not be possible to enter into a more complete description of the diffi- 

 cult details of the horopter. For the determination of the horopter, v. Helmholtz 

 constructs, in the primary position, similar meridians and parallel circles over 

 both retinas; the identical points lie then as if on two globes of equal length and 

 breadth. Hering draws two systems of planes through the eyeballs in the primary 

 position: those of one system (the transverse) intersect in the transverse axis 



fi 



FIG. 303. Diagrammatic Representation of Identical FIG. 304. Horopter for the Secondary Posi- 



and Nonidentical Retinal Points. tion, with Convergence of the Visual Axes. 



connecting the nodal points of the eyeballs. Those of the second system inter- 

 sect in a perpendicular drawn through the nodal point of each eye. The identical 

 points he at the intersections of the similar perpendicular and transverse planes 

 of the retinas. 



All objects whose rays fall on nonidentical (disparate) points of the 

 retinas appear in double images. A distinction is made between homony- 

 mous and crossed (heteronymous) double images, according as the rays 

 from the disparate retinal points intersect in front of or behind the 

 fixed point. 



In illustration, two fingers are held, one behind the other, in front of the eyes, 

 it the tar finger is looked at, the other appears double, while if the near one is 

 fixed, the far one appears double. If, while looking at the far finger, the right 

 eye is closed the left (crossed) image of the first finger disappears. If the near 

 finger is fixed and the right eye is closed, the right (homonymous) image of the 

 second finger disappears. 



The double images, like the single images, are referred to the proper distance 

 by the eyes. 



