W. B. Rogers on Binocular Vision. 89 
cacageee image placed between the two lateral ones at or very 
near By giving the een a slight vibratory motion around the 
end a we cause the pin rto pass alternately to the right and 
left at a small distance behind or before the i image, while the lat- 
ter is seen to maintain its position at the point of intersection of 
_ the optic axes. By making r the centre of the vibration we pro- 
duce a like movement of r in regard to the image. 
In either of the modes of adjustment which have been de- 
scribed this instrument is be used as a stereoscope. It is true 
that those observers, who by any practise have acquired the power 
of convergiug their optic axes steadily upon near and remote 
combine the corresponding pictures and obtain the fu 
scopic effect. But most persons find ong difficulty: in| 
ing the necessary adjustment, and n istatice of some 
guide-object as the pin r in a proper vould 
By placing stereoscopic drawings of suitable dimensions in an 
Sarighe position against the pins oe (fig./1), and directing the 
eyes to or beyond the remote pi in this case should be 
taller than the others, we see. ‘T Meultant perspective image at 
the place to which the axes are converged ; or if we rest the 
cae against a (fig. 2), bth converge the eyes tor or to 
S it readily fentbd: before or behind it, we see the result- 
ant image perfectly formed in that position. It is hardly neces- 
ry to say that, in the former case, the points of the twin — 
which are to be united, should be at a less distance apart t 
that which separates the centres of the eyes. When the liathiil 
is exactly equal to ab (fig. 1), the image will be formed at or 
nearer the eyes; ed less it must be i pie a b. 
2. Of the apparent distance and magnitude of the lateral or 
1 gate aea jigures. 
~ 
in the latter than the components. But if we fix the eyes 
Sroonn Sears, Vol. XX, No. itis 1855. 12 
