Prof. Wheatstone on the Physiology of Vision. 247 



ting the screw r I and fixing the upright boards D D' at the 

 proper distances. The sliding pannels may also be dispensed 

 with, and the drawings themselves be made to slide in the grooves. 



§4. 



A few pairs of outline figures, calculated to give rise to the 

 perception of objects of three dimensions when placed in the 

 stereoscope in the manner described, are represented in PI. VIII. 

 figs. 10 to 20. They are one half the linear size of the figures 

 actually employed. As the drawings are reversed by reflexion in 

 the mirrors, I will suppose these figures to be the reflected images 

 to which the eyes are directed in the apparatus ; those marked 

 b being seen by the right eye, and those marked a by the left 

 eye. The drawings, it has been already explained, are two dif- 

 ferent projections of the same object seen from two points of 

 sight, the distance between which is equal to the interval be- 

 tween the eyes of the observer ; this interval is generally about 

 2~ inches. 



a and b, fig. 10, will, when viewed in the stereoscope, present 

 to the mind a line in the vertical plane, with its lower end in- 

 clined towards the observer. If the two component lines be 

 caused to turn round their centres equally in opposite directions, 

 the i*esultant line will, while it appears to assume every degree 

 of inclination to the referent plane, still seem to remain in the 

 same vertical plane. 



Fig. 11. A series of points all in the same horizontal plane, 

 but each towards the right hand successively nearer the observer. 



Fig. 12. A curved line intersecting the referent plane, and 

 having its convexity towards the observer. 



Fig. 13. A cube. 



Fig. 14. A cone, having its axis perpendicular to the referent 

 plane, and its vertex towards the observer. 



Fig. 15. The frustum of a square pyramid; its axis perpen- 

 dicular to the referent plane, and its base furthest from the eye. 



Fig. 16. Two circles at different distances from the eyes, their 

 centres in the same perpendicular, forming the outline of the 

 frustum of a cone. 



The other figures require no observation. 



For the purposes of illustration I have employed only outline 

 figures, for had either shading or colouring been introduced it 

 might be supposed that the effect was wholly or in part due to 

 these cirCUmitUlOSSj whereas by leaving them out of considera- 

 tion no room is lefl to doubt that the entire effect of relief is 

 owing to the simultaneous perception d|' the two monocular pro- 

 jection-, one mi each retina. Hut if it be required to obtain tile 



most faithful resemblances of real objects, shadowing and co- 



