Prof. Wheatstone on the Physiology of Vision. 513 



another pair of projections ; but, by means of tbe stereoscope, 

 we have it witbin our power to associate these circumstances 

 abnormally, and to cause any degree of inclination of the axes to 

 coexist with any dissimilarity of the two pictures. To ascertain 

 experimentally what takes place under these circumstances, 

 M. Claudet prepared for me a number of Daguerreotypes of the 

 same bust, taken at a variety of different angles, so that I was 

 enabled to place in the stereoscope two pictures taken at any 

 angular distance from 2° to 18°, the former corresponding with 

 a distance of about 6 feet, and the latter with a distance of about 

 8 inches. The effect of a pair of near projections seen with a 

 distant convergence of the optic axes, is to give an undue elon- 

 gation to lines joining two unequally distant points, so that all 

 the features of a bust appear to be exaggerated in depth. The 

 effect, on the contrary, of a pair of distant projections, seen with 

 a near convergence of the axes, is to give an undue shortening 

 to the same lines, so that the appearance of a bas-relief is ob- 

 tained from the two projections of the bust. The apparent di- 

 mensions in breadth and height remain in both cases the same. 



§21. 



To reproduce the conditions of the binocular vision of a soUd 

 object as completely as possible by means of its two plane pro- 

 jections, it is necessary, as I have before stated, that the projec- 

 tions shall be such as correspond exactly with the inclination of 

 the optic axes under which they are viewed. I have already 

 shown in § 20 what takes place when this condition is not 

 strictly observed, and I may add, that the mind is not unplea- 

 santly affected by a considerable incongruity in this respect ; on 

 the contrary, the effect in many cases seems heightened by view- 

 ing the solid appearance, intended for a determinate degree of 

 inclination of the axes, under an angle several degrees less ; the 

 reality is as it were exaggerated. When the optic axes are 

 parallel, in strictness there should be no difference between the 

 pictures presented to each eye, and in this case there would be 

 no binocular relief; but I find that an excellent effect is pro- 

 duced when the axes are nearly parallel by pictures taken at an 

 inclination of 7° or 8°, and even a difference of 16° or 17° has 

 no decidedly bad effect. 



This circumstance enables us to combine the ideal amplifica- 

 tion arising from viewing pictures placed near the eyes under a 

 small inclination, or even parallelism, of the optic axes mentioned 

 in § 17, with the perception of solidity arising from the dissimi- 

 larity of the projections ; for this purpose, the pictures in the 

 refracting stereoscope, or their reflected images in the reflecting 

 instrument, must be viewed through lenses the focal distance of 



Phil. May. S. 4. No. 21. Suppl. Vol. 3. 2 L 



