356 



THE SPECIAL SENSES. 



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 byreason 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. 153. 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 

 nature or by an artifice, we at once perceive depth or solidity in the 

 object. This fact is made use of in all devices employed to produce 

 stereoscopic vision. 



Stereoscopic Vision. Stereoscopic pictures may be obtained 



by photographing the 

 same object or collec- 

 tion of objects from 

 slightly different 

 points so as to get a 

 right-eyed and a left- 

 eyed picture ; or for 

 simple outline pic- 

 tures, such as geo- 

 metrical figures, they 

 may be made by draw- 

 ings of the object as seen by the two eyes, respectively (see Figs. 

 153 and 155). Any optical device that will enable us to throw the 



Fig. 153. Right- and left-eyed images of truncated 

 pyramid. May be combined to produce solid image by 

 relaxing the accommodation, that is, gazing to a dis- 

 tance through the book. 



