XX VIIL—IMPROVEMENTS IN THE STEREOSCOPE, sy 
HOWARD GRUBB, m.£., F.R.A.s. Puates 10 and 11. 
[Read January 20th, 1879.] 
WHEN we possess a pair of properly taken stereoscopic pictures, all 
that is necessary to utilize these in producing that effect which 
we call “stereoscopic” is some optical contrivance which will 
enable us to view one picture with one eye simultaneously with, 
and under the same conditions as, the other picture, viewed was 
the other eye. 
This seems simple enough, and yet since the first invention of 
the first instrument, in 1838, only one other has come into general 
use, 
Of these two instruments, the first was invented by Sir Chas. 
Wheatstone in 1838, and is generally known as the Reflecting 
Stereoscope of Wheatstone ; the second was invented by Sir David 
Brewster in 1849, and is generally known as the Lenticular 
Stereoscope of Brewster. 
The first, or Reflecting Stereoscope of Wheatstone, is the most 
perfect instrument ; but the second, or Lenticular Stereoscope of 
Brewster, though inferior in many respects, completely beat its 
rival out of the field by reason of its compactness, convenience, 
and cheapness. In fact, now-a-days the Wheatstone Stereoscope 
is hardly known. Before describing the various forms of stereo- 
scopes brought before the public at various times, and some new 
forms, I now, for the first time, venture to bring forward, it may 
be desirable to consider a little the theory of the stereoscope, and 
thus, perhaps, arrive at a more correct idea of what conditions 
we require to fulfil. 
To go thoroughly into the theory of the stereoscope, we should 
consider the various theories of binocular vision, in which there 
are still some points which physicists find it hard to come to an 
agreement about. While some have shown that we can really only 
direct our attention to, and see distinctly, one single point of a 
subject at one time, and have argued therefrom that our general 
impression of a subject is derived from an enormously rapid sur-' 
vey taken of each point seriatim, Prof. Tyndall has, it appears 
ScrEN. Proc. R.D.S., Vou. 1, Pr. 111. oO 
