550 VISUAL JUDGMENTS. [BOOK in. 



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 subjective 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 monocular judgments of distance 

 must be subject to much error. Everyone 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 shall result. When the 

 object is near, we converge our visual axes ; when distant, we bring 

 them back towards 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 necessary to bring the object into 

 single vision is lessened, the object seems to become more distant, 

 when increased, to move towards us: as may be seen in the 

 stereoscope. 



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

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

 farther than the apparent size of the image, i. e. of the angle sub- 

 tended 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 distance 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 delusion. So also an image on a screen 

 when gradually enlarged seems to come forward, when gradually 



