510 Prof. Wheatstone on the Physiology of Vision. 



I have, for many years past, employed also another means to 

 occasion, without any straining of the eyes, the coincidence of 

 the pictures so that the image in relief shall appear of the same 

 magnitude and at the same distance as the object which they 

 represent would do if it were itself directly regarded. In this 

 apparatus, prisms 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 stereoscope. 



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 partition 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 aper- 

 tures are fixed a pair of glass prisms having their faces inclined 

 1 5°, 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 coin- 

 cidence 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 some- 

 what 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. 



Stei'eoscopic drawings afford a means of illustrating works 

 with figures of three dimensions, instead of with 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 projections. For this purpose 

 the corresponding binocular figures must be engraved in parallel 

 vertical columns, and their coalescence may be effected by view- 

 ing them through a pair of prisms, similar to those employed in 

 the refracting stereoscope, placed in a frame at the proper di- 



