VISUAL JUDGMENTS. 787 



object distinct, we are led to form judgments, i. e., to draw conclusions con- 

 cerning the external world by means of an interpretation of our visual per- 

 ceptions. Looking before us, we say we see a certain object of a certain 

 color nearly in front of us or much on our right hand or much on our 

 left ; that is to say, we judge such an object to be in such a position because 

 from the constitution of our brain, strengthened by all our experience, we 

 associate such a part of our field of vision with such an object. The sub- 

 jective visual complex sensation or perception is to us a symbol of the 

 external object. 



Even with one eye we can, to a certain extent, form a judgment, not only 

 as to the position of the object in a plane at right angles to our visual axis, 

 but also as to its distance from us along the visual axis. If the object is 

 near to us, we have to accommodate for near vision ; if far from us, to relax 

 our accommodation mechanism so that the eye becomes adjusted for distance. 

 The muscular sense (of which we shall speak presently) of this effort enables 

 us to form a judgment whether the object is far or near. Seeing the narrow 

 range of our accommodation, and the slight muscular effort which it entails, 

 all molecular judgments of distance must be subject to much error. Every- 

 one who has tried to thread a needle without using both eyes knows how 

 great these errors may be. When, on the other hand, we use two eyes, we 

 have still the variations in accommodation, and, in addition, have all the 

 assistance which arises from the muscular effort of so directing the two 

 eyes on the object that single vision will result. When the object is near, 

 we converge our visual axes ; when distant, we bring them back toward 

 parallelism. This necessary contraction of the ocular muscles affords a 

 muscular sense, by the help of which we form a judgment as to the distance 

 of the object. Hence, when by any means the convergence which is neces- 

 sary to bring the object into single vision is lessened, the object seems to 

 become more distinct, when increased, to move toward us as may be seen 

 in the stereoscope. 



689. The judgment of size is closely connected with that of distance. 

 Our perceptions, gained exclusively from the field of vision, go no further 

 than the apparent size of the image, i. e., of the angle subtended by the 

 object. The real size of the object can only be gathered from the apparent 

 size of the image when the distance of the object from the eye is known. 

 Thus perceiving directly the apparent size of the image, we judge the dis- 

 tance of the object giving the image, and upon that come to a conclusion as 

 to its size. And, conversely, when we see an object, of whose real size we 

 are otherwise aware, or are led to think we are aware, our judgment of its 

 distance is influenced by its apparent size. Thus when in our field of vision 

 there appears the image of a man, knowing otherwise the ordinary size of a 

 man, we infer, if the image be very small, that the man is far off. The 

 reason of the image being small may be because the man is far off, in which 

 case our judgment is correct ; it may be, however, because the image has 

 been lessened by artificial dioptric means, as when the man is looked at 

 through an inverted telescope, in which case our judgment becomes a delu- 

 sion. So also an image on a screen when gradually enlarged seems to come 

 forward, when gradually diminished seems to recede. In these cases the 

 influence on our judgment of the muscular sense of binocular adjustment, or 

 monocular accommodation, is thwarted by the more direct influence of the 

 association between size and distance. 



690. Judgment of solidity. When we look at a small circle, all parts 

 of the circle are at the same distance from us, all parts are equally distinct 

 at the same time, whether we look at it with one eye or with two eyes. 

 When, on the other hand, we look at a sphere, the various parts of which are 



