SOME ASPECTS OF STEREOPS1S 97 



depth of the frustum, as well as each of the edges nearest the obser- 

 ver, is s , the edges of the other face must be 



s(s + d)/(Ms + d) =s[l -us/d] , 



if Ms is small by comparison with d . Note that this result is inde- 

 pendent of // . 



While no mechanism is suggested here for this dependence of 

 perceived distance upon apparent size, we note that a converse mech- 

 anism has been suggested by Landahl (1939a) to account for the 

 constancy of perceived size with the concomitant (mutually inverse) 

 variation of apparent size and actual distance. 



The factor of accommodation (Stanton, 1942) is certainly not 

 sensitive as a distance-cue, but there is evidence that it does operate 

 (Grant, 1942). Distant objects are seen with the relaxed eye (if em- 

 metropic), whereas to focus clearly on nearby objects requires an 

 effort of accommodation. Convergence, a binocular cue, is more certain 

 (Householder, 1940c). Convergence upon near objects also require 

 an effort, although it is known that the visual axes of relaxed eyes are 

 not parallel, but diverge, so that some effort is required also for bi- 

 nocular vision at a distance. The cues of both accommodation and 

 convergence are muscular, and neither is sensitive enough to provide 

 the fine discrimination known to be possible in binocular stereopsis. 

 In fact, binocular stereopsis can be achieved by means of a stereo- 

 scope when the visual axes are parallel and accommodation is relaxed. 

 Thus other cues must be sought. 



In normal binocular vision whereas there are two retinal images, 

 one in each eye, of the object fixated, there is but one cortical image 

 — the individual sees but one object. On the other hand if the atten- 

 tion, but not the fixation, is shifted to an object enough nearer or far- 

 ther than the point of fixation, then two images of this single object 

 are seen. Objects somewhere in between the point of fixation and the 

 object seen doubly may be seen singly but, if so, they can be recog- 

 nized as nearer to or farther from the observer, as the case may be, 

 than the point of fixation. 



Let C L and C R represent the centers of rotation of the left and right 

 eyes, respectively. The separation between nodal point and center 

 of rotation is so slight that for present purposes we may regard C L 

 and C R as being also the nodal points. Let P represent the fixation- 

 point. Then PC L and PC R are the two visual axes; let these cut the 

 retinas at P L and P R . The two retinal images of P are therefore lo- 

 cated at P L and P R , and when so located these images fuse and the 

 point P is seen singly. Let P be any point on the line C L P be- 

 tween C L and P . Then P is also imaged at P L on the left retina, but 



