64:0 THE SENSES. 



the nearer one is no longer in the point of fixation. For the right eye, 

 its image will appear to the left of the line of direct vision, and for the 

 left eye to the right of this line. It therefore appears double and in- 

 distinct. 



Thus, in the ordinary use of binocular vision every object but one 

 appears double and at the same time imperfectly delineated. This cir- 

 cumstance is so little noticed that it is never a source of confusion for 

 the sight, and even requires a special experiment to demonstrate its 

 existence. The reason for its passing, as a general rule, unobserved is 

 twofold. First, the attention is naturally concentrated upon the object 

 which is placed, for the moment, at the point of fixation. When this 

 point is shifted, the new object upon which it falls also appears single ; 

 and thus the idea of a double image, even if indistinctly suggested at 

 any time, is at once dispelled by the movement of the eyes in that 

 direction. Secondly, an object which is really placed in any degree 

 toward the right hand or the left will form an indistinct double image, 

 since it occupies a different apparent position for the two eyes. But 

 the obliquity of its rays, and consequently the indistinctness of its 

 image, will be greater for the right eye than for the left, or vice versa; 

 and the notice of the observer, if drawn to it at all, is occupied with 

 the more distinct of the two images, to the exclusion of the other. 

 The fact becomes palpable only in such an experiment as that detailed 

 above ; where two bodies are examined in the same linear range, so that 

 the double images produced are equal in intensity, and sufficiently de- 

 tached by contrast from surrounding objects to force themselves upon 

 the attention. 



Double vision may also be produced at any time by pressure with 

 the finger at the external angle of one of the eyes, so as to alter its posi- 

 tion in the orbit, the other eye remaining untouched. But in this case 

 it is the whole field of vision which is displaced, and all objects are 

 doubled indiscriminately ; their images being separated to the same 

 degree and in the same direction, whatever may be their distance from 

 the eye. It is this form of double vision which is produced, in vertigo 

 or intoxication, by irregular action of the muscles of the eyeball. 



Appreciation of Solidity and Projection. When both eyes are direct- 

 ed simultaneously at a single point, the distance of the object may be 

 estimated with considerable accuracy by the degree of convergence of 

 the visual axes required for its fixation. Since this convergence is 

 in proportion to the proximity to the observer of the point of fixation, 

 another impression, of different kind but of equal importance, is also 

 produced by binocular vision, when the object has an appreciable volume 

 and thickness, and when it is placed within a moderate distance. Owing 

 to the lateral separation of the two eyes, and the convergent direction 

 of their visual axes, they do not both receive from such an object pre- 

 cisely the same image. Both e} r es will see the front of the object in 

 nearly the same manner ; but in addition the right eye will see a little 

 of its right side, and the left eye will see a little of its left side. This 



