376 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



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 repre- 

 sented from figs. 10. to 20. They are one half the linear size of the figures actually em- 

 ployed. As the drawings are reversed by reflection in the mirrors, I will suppose 

 these figures to be the reflected images to which the eyes are directed in the appa- 

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

 eye. The drawings, it has been already explained, are two different projections of the 

 same object seen from two points of sight, the distance between which is equal to the 

 interval between the eyes of the observer ; this interval is generally about 2 J 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 inclined towards the observer. If 

 the two component lines be caused to turn round their centres equally in 

 opposite directions, the resultant 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 con- 

 vexity 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 perpendicular 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 circumstances, whereas by leaving them out of consideration 

 no room is left to doubt that the entire effect of relief is owing to the simultaneous 

 perception of the two monocular projections, one on each retina. But if it be re- 

 quired to obtain the most faithful resemblances of real objects, shadowing and co- 

 louring may properly be employed to heighten the effects. Careful attention would 

 enable an artist to draw and paint the two component pictures, so as to present to the 

 mind of the observer, in the resultant perception, perfect identity with the object re- 

 presented. Flowers, crystals, busts, vases, instruments of various kinds, &c., might 

 thus be represented so as not to be distinguished by sight from the real objects them- 

 selves. 



