6 
MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
being employed to deflect the rays of light proceeding from the pictures, so as to 
make them appear to occupy the same place, I have called it the refracting stereo- 
scope. 
It is represented by fig. 4. It consists of a base 6 inches long and 4 inches broad, 
upon which stands an upright partition, 5 inches high, dividing it equally ; this par- 
tition is capable of extension by means of a slide to double the length, and carries at 
its upper extremity a board placed parallel to the base, and of the same dimensions. 
In this upper board there are two apertures an inch square, one on each side of the 
partition, the centres of which are 2^ inches from each other ; in these apertures are 
fixed a pair of glass prisms having their faces inclined 15°, and their refractive angles 
turned towards each other. The stereoscope pictures are to be placed on the base, 
and their centres ought not to exceed the distance of 2| inches. 
A pair of plate-glass prisms, their faces making with each other an angle of 12°, 
will bring two pictures, the corresponding points of which are 2^ inches apart, to 
coincidence at a distance of 12 inches, and a pair with an angle of 15° will occasion 
coincidence at 8 inches. 
The refracting stereoscope has the advantage of portability, but it is limited to 
pictures of small dimensions. It is well suited for Daguerreotypes, which are usually 
of small size, and, on account of the nature of their reflecting surface, must be viewed 
in a particular direction with respect to the light which falls upon them ; whereas in 
the reflecting stereoscope it is somewhat difficult to render the two Daguerreotypes 
equally visible. For drawings and Talbotypes it however offers no advantages, 
though it is equally well suited for them when their dimensions are small. 
Stereoscopic drawings afford a means of illustrating works with figures of three 
dimensions, instead of wdth mere plane representations. Works on crystallography, 
solid geometry, spherical trigonometry, architecture, machinery, &c., might be thus 
rendered more instructive, from the perfect counterpart of the solid figure seen from 
a single point of view being represented, instead of merely one of its plane projec- 
tions. For this purpose the corresponding binocular figures must be engi-aved in 
parallel vertical columns, and their coalescence may be effected by viewing them 
through a pair of prisms, similar to those employed in the refracting stereoscope, placed 
in a frame at the proper distance from each other. If the engravings should be less 
than 2^ inches apart, the prisms may be dispensed with by persons who have com- 
mand over the adaptation of their eyes, particularly if they be short-sighted. 
^ 19. 
At the date of the publication of my experiments on binocular vision, the brilliant 
photographic discoveries of Talbot, Niepce and Daguerre, had not been announced 
to the world. To illustrate the phenomena of the stereoscope I could therefore, at 
that time, only employ drawings made by the hands of an artist. Mere outline 
figures, or even shaded perspective drawings of simple objects, do not present much 
