Improvements in the Stereoscope. 181 
case ; so that in the first case the point b b' appears to be far below 
the point a a’, and in the second as much above. 
Besides this power we have of judging of the distances of 
objects by the convergence of the optic axes, we have evidently 
other, though much less delicate means, of doing so. 
If we shut one eye we can still appreciate, to a slight extent, 
relative distances. How very much reduced, however, our power 
is in this respect will be evident from a trial of the old experiment, 
of walking up to a candle with one eye shut and trying to snuff 
the wick. 
Much of this remaining power is probably due to the diameter 
of the pupil of the eye. If this be reduced by diaphragms, our 
sense of relief or perception of form in three dimensions is much 
reduced, just as a photograph taken with a small aperture of 
lens has none of that roundness and relief that is admired so 
much in photographs taken with large aperture lenses, This 
was pointed out by my father about twenty years since. 
There still remains even after shutting one eyeand diaphragming 
the other down, some slight appreciation of distance, quite 
apart from intuitive ideas of perspective light and shade, or 
such matters. This may partly be due to differences of focus, 
and also (to a larger extent, perhaps, than we would imagine) 
to the fact that we are familiarized with most of the surrounding 
objects and their actual size, and are therefore in a position to 
judge of their distance from their angular size. Some of these 
points may be useful further on in considering some forms of 
stereoscope. 
Wheatstone Stereoscope-—The principle of the Wheatstone 
Reflecting Stereoscope is easily understood. 
The pictures are placed (Fig. 2, Plate 10) opposite and facing 
one another, at about double the distance at which it is desired to 
view them. Midway between the pictures are placed a pair of 
reflectors, so inclined that the right eye sees the right picture, and 
the left eye sees the left picture, superposed on the image cf the 
other. There is a difference of opinion about the best position of 
the virtual images, whether they should be superposed or side by 
side, but this I will refer to further on. 
The Wheatstone Stereoscope is theoretically and practically a 
more perfect instrument than that in ordinary use, but the 
SciEN, Proc. R.DS., Vou. m, Pr. 111. 0? 
