346 THE SPECIAL SENSES. 



would have on the retina a projection of this kind /\ are with the lens projected 

 inverted \/, and our visual judgments are controlled by this factor in spite 

 of the opposing evidence from the size of the retinal images. In order for 

 the experiment to succeed it is necessary that the objects viewed shall be far 

 enough away so that a flat picture may be given by the lens, that is, a pic- 

 ture in which the foci for the near points shall not differ practically from those 

 of more distant points, otherwise the muscular movements of accommodation 

 interfere with the delusion. The relative importance of this last factor 

 (see succeeding paragraph) is well illustrated by varying the experiment in 

 this way: Place two objects upon a well-lighted table, one at the near end 

 and one at the far end. Then standing close to the table view these objects 

 through the lens as before. They will be seen in their right relations to each 

 other. If, however, one backs away from the table while watching the images 

 there will come a distance at which the near object will be seen to shift around 

 to the rear of the far object. 



(c) The Muscle Sense (Focal Adjustment). For objects near 

 enough to require accommodation it is obvious that the nearer 

 object will need a stronger contraction of the ciliary muscle, and 

 also of the internal rectus in order to bring the line of sight to bear 

 correctly. By means of the fibers of muscle sense we have a very 

 exact conception of the degree of contraction of these muscles, and 

 this sensation is perhaps the most important factor used in making 

 our monocular judgments of depth for objects at a short distance. 

 In binocular vision the same factor is doubtless of increased effi- 

 ciency by reason of the sensations obtained from the two eyes. 



(d) The disposition of lights and shades and the size of familiar 

 objects. It may be assumed that in distant vision of complex 

 fields the varying lights and shades exhibited by objects according 

 as they stand in front of or behind each other also aid our judg- 

 ment. The actual size also of the retinal images of familiar objects 

 such as animals, trees, etc. gives us an accessory fact which con- 

 tributes to the impression derived from the sources mentioned 

 above. These factors are employed with effect by the artist in 

 strengthening the general impression which he wishes to give of 

 the difference between the foreground and the background. 



The Binocular Perspective. In binocular vision there is an 

 additional element which contributes greatly to our judgment of 

 depth. This element consists in the fact that the retinal images 

 of external objects, particularly near objects, are different in the two 

 eyes. Inasmuch as the eyes are separated by some distance the 

 projection of any solid object upon one retina is different from 

 the projection on the other. If a truncated pyramid is held in 

 front of the eyes, the right eye sees more of the right side, the left 

 more of the left side. The projection of the same object upon the 

 two retinas may, in fact, be represented by the drawings given in 

 Fig. 150. Whenever this condition prevails, whenever what we 

 may call a right-eyed image of an object is thrown on the right eye 

 and simultaneously a left-eyed image on the left eye, whether in 



