SINGLE VISION. 459 



of this convergence, the mind certainly acquires, by experience, dis- 

 tinct cognizance, and hence refers both images to one object, as only 

 one object can occupy the same point in space. In certain cases, as 

 in squinting suddenly produced, double vision for a time occurs, but 

 afterwards the mind neglects one or other image ; so, too, of the 

 images of the same objects, which fall on non-corresponding points 

 of the retinae, one is either neglected, or becomes obscured by the 

 stronger impression in the other eye. Doubtless, also, we are con- 

 stantly neglecting double images formed, in the two eyes, of objects 

 around the point of single vision. 



It was shown by Mu'ller, that when the eyes are fixed on a certain 

 point, it is only those objects lying on a curved line, the chord of 

 which is formed by the distance between the two eyes, or rather be- 

 tween the points of decussation of the luminous rays in the two eyes, 

 which appear single. This curved line is named the horopter (Agui- 

 lonius), and its size and curvature are determined by three points, 

 viz., the centres of the two eyes, and the points towards which the 

 axes of these converge. By Helmholz, however, the horopter is shown 

 to be usually a line of double curvature, formed by the meeting of two 

 hyperbolic, or sometimes of two plane curves. Moreover, when the 

 point of convergence of the eyes is in the median plane of the head, 

 and at an infinite distance, then the horopter is really a plane, parallel 

 to the two visual lines, and corresponds with the ground on which we 

 stand. In near vision, objects exactly in the horopter, are better seen 

 stereoscopically, than those out of it, as may be illustrated by looking 

 at a wire slightly bent, in its middle, towards the eyes, and held in 

 front of the face, first out of, and then in, the line of the horopter, 

 when it will be found that the bend in the wire, is most easily seen in 

 the latter position. So, also, in distant vision, the surface of the 

 landscape, and the distance of its several points, are better estimated, 

 because they lie in the plane of the horopter; for, if the head be 

 turned aside, or inverted, our perception of those distances is less ac- 

 curate. 



In viewing an object situated beyond a certain distance, the con- 

 vergence of the visual axes is no longer necessary, and it has been 

 calculated that these remain parallel for all objects, the distance of 

 which exceeds 120 feet. The angle formed by the lines of vision of 

 the two eyes at the object, is called the binocular parallax of the ob- 

 ject; for objects at a distance of 12 feet, this angle measures about 1 ; 

 it is, of course, regulated, for each individual, by the interocular dis- 

 tance. 



The objects which we have to examine in external nature, are 

 bodies having three dimensions, viz., length, breadth, and depth ; in 

 other words, they occupy space, and possess solidity, however varied 

 their shape, whether cubical, oblong, cylindrical, ovoid, or spherical, 

 whether convex or concave, regular or irregular. In estimating the 

 forms of such bodies with the aid of one eye only, we are guided by 

 the ascertained correspondence of certain effects of light and shade 

 with certain constant impressions derived from touch. Experience 

 alone enables us to form complex notions, or judgments, of this kind. 



