MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
7 
difficulty; but it is evidently impossible for the most accurate and accomplished 
artist to delineate, by the sole aid of his eye, the two projections necessary to form 
the stereoscopic relief of objects as they exist in nature with their delicate dilferences 
of outline, light and shade. What the hand of the artist was unable to accomplish, 
the chemical action of light, directed by the camera, has enabled us to effect. 
It was at the beginning of 1839, about six months after the appearance of my me- 
moir in the Philosophical Transactions, that the photographic art became known, and 
soon after, at my request, Mr. Talbot, the inventor, and Mr. Collen (one of the first 
cultivators of the art) obligingly prepared for me stereoscopic Talbotypes of full-sized 
statues, buildings, and even portraits of living persons. M. Quetelet, to whom I 
communicated this application and sent specimens, made mention of it in the Jlulletins 
of the Brussels Academy of October 1841. To M. Fizeau and M. Claudet I was 
indebted for the first Daguerreotypes executed for the stereoscope. The beautiful 
stereoscopic representations of statuary, architecture, machinery, natural history spe- 
cimens, portraits of living persons, single and in groups, &c., which have recently 
been produced by M. Soleil and M. Claudet, are now too well known to the public 
to need more than a slight reference to them. 
With respect to the means of preparing the binocular photographs (and in this 
general term I include both Talbotypes and Daguerreotypes), little requires to be said 
beyond a few directions as to the proper positions in which it is necessary to place 
the camera in order to obtain the two required projections. 
We will suppose that the binocular pictures are required to be seen in the stereo- 
scope at a distance of 8 inches before the eyes, in which case the convergence of the 
optic axes is about 18°. To obtain the proper projections for this distance, the 
camera must be placed, with its lens accurately directed towards the object, suc- 
cessively in two points of the circumference of a circle 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 arbitrarily, 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. Miniature stereoscopic repre- 
sentations of buildings and full-sized statues are therefore obtained merely by taking 
the two projections 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 
