512 Prof. Wheatstone on the Physiology of Vision. 



of which the object is the centre, and the points at which the 

 camera is so placed must have the angular distance of 18° from 

 each other, exactly that of the optic axes in the stereoscope. 

 The distance of the camera from the object may be taken arbi- 

 trarily; for, so long as the same angle is employed, whatever 

 that distance may be, the pictures will exhibit in the stereoscope 

 the same relief, and be seen at the same distance of 8 inches, 

 only the magnitude of the picture will appear different. Minia- 

 ture stereoscopic representations of buildings and full- sized 

 statues are therefore obtained merely by taking the two projec- 

 tions of the object from a considerable distance, but at the same 

 angle as if the object were only 8 inches distant, that is, at an 

 angle of 18°. 



To produce the best effect, it is necessary that the pictures be 

 so placed in the stereoscope that each eye shall see its respective 

 picture at the proper point of sight : if this condition be not 

 attended to, the binocular perspective will be incorrect. 



For obtaining binocular photographic portraits, it has been 

 found advantageous to employ, simultaneously, two cameras fixed 

 at the proper angular positions. 



I subjoin a Table of the inclinations of the optic axes which 

 correspond to different distances; it also shows the angular 

 positions of the camera required to obtain binocular pictures 

 which shall appear at a given distance in the stereoscope in their 

 true relief. 



Inclination of the optic axes 2° 

 Distance in inches 71 - 5 



6° 8° 



23-8 17-8 



a 6 



The distance is equal to 5 cotang 5 ; a denoting the distance 



between the two eyes, and 6 the inclination of the optic axes. 



§20. 



As the inclination of the optic axes diminishes by the removal 

 of an object to which they are directed to a greater distance, not 

 only does the magnitude of the pictures projected by it on the 

 retina? proportionately diminish, but the dissimilarity of the pic- 

 tures becomes less. The difference of distance between any two 

 points of each of the pictures will diminish until the projections 

 become sensibly similar. Under the usual circumstances attend- 

 ing the vision of a solid object placed at a given distance, a par- 

 ticular inclination of the axes is invariably accompanied by a 

 specific pair of dissimilar projections; and if the distance be 

 changed, a different inclination of the axes is accompanied by 



