7 38 THE HOROPTER. 



Mention appear to be vertical to those on the horizontal lines, although they are not actually 

 M^Henoe the vertical lines of separation are the apparent vertical meridians. Some 

 observers reirard the identical points of the retina as an acquired arrangement ; others regard it 

 aTnarmaUy innate. Persons who have had a squint from their birth see singly; in these 

 cases, the identical points must be ditferently disposed. 



The 'horopter represents all those points of the outer world from which rays of 



B 



A 



Fig. 567. 



Scheme of identical and non-identical points 

 of the retina. 



Fig. 568. 



Horopter for the secondary position, with 



convergence of the visual axes. 



light passing into both eyes fall upon identical points of the retina, the eyes being 

 in a certain position. It varies with the different positions of the eyes. 



1. In the primary position of both eyes with the visual axes parallel, the rays of direction 

 proceeding from two identical points of the two retinae are parallel and intersect only at 

 infinity. Hence for the primary position the horopter is a plane in infinity. 



2. In the secondary position of the eyes with converging visual axes, the horopter for the 

 transverse lines of separation is a circle which passes through the nodal points of both eyes (fig. 

 568, K, K), and through the fixed points I, II, III. The horopter of the vertical, lines of 

 separation is in this position vertical to the plane of vision. 



3. In the symmetrical tertiary position, in which the horizontal and vertical lines of 

 separation form an angle, the horopter of the vertical lines of separation is a straight line 

 inclined towards the horizon. There is no horopter for the identical points of the horizontal 

 lines of separation, as the lines of direction prolonged from the identical points of these points 

 do not intersect. 



4. In the unsymmetrical tertiary position (with rolling) of the eyes, in which the fixed point 

 lies at unequal distances from both nodal points, the horopter is a curve of a complex form. 



All objects, the rays proceeding from which fall upon non-identical points oi the 

 retina;, appear double. We can distinguish direct or crossed double images, accord- 

 ing as the rays prolonged from the non-identical points of the retina intersect in 

 front of or btliind the fixed point. 



Experiment. Hold two fingers the one behind the other before both eyes. Accommo- 

 date for the far one and then the near one appears double, and when we accommodate for the 

 near one the far one appears double. If, when accommodating for the near one, the right eye 

 be closed, the left (crossed) image of the far finger disappears. On accommodating for the 

 far finger and closing the right eye, the right (direct) double image of the near finger 

 disappears. 



Double images are referred to the proper distance from the eyes, just as single 

 images are. 



