180 Scientific Proceedings, Royal Dublin Society. 
to me, shown conclusively that a general idea of the relative 
positions and distances of all the salient features of the scene can 
be appreciated without actually directing the eye in succession to 
every individual point. Jn his own words :— 
‘«‘« When the optic axes are converged upon a certain point of an object 
the other points produce a certain determinate effect on the retina, and 
are in some measure the objects of our attention. There is thus estab- 
lished an association between a certain convergence of the optic axes and 
certain incidental impressions, and this association may, I think, become 
so refined by habit as to enable us to infer the solidity of a body, or the 
relative distances of objects; while the optic axes are kept immovably 
fixed on a single point.” 
We shall probably have to refer to a somewhat similar matter 
as we proceed ; for our present purpose, however, it will not be 
necessary to touch further on these debated points. 
In order to understand the principles of the stereoscope, it will 
only be necessary to assume that the nearer an object is to us the 
greater convergence we must give to the optic axes, in order to 
view this object with both eyes; and, as a consequent upon this 
last, that if, to see any point in a subject, we are obliged to give a 
greater convergence to the optic axes than for the other points, we 
intuitively assume that point to be nearest to us. We need hardly 
stop to prove this ; a moment’s consideration will show how im- 
possible it is it could be otherwise. Now, this being so, if we place 
a stereoscopic diagram of a truncated hexagon in an ordinary stereo- 
scope (Fig. 1, Plate 10), we appear to be looking into a hollow pyra- 
mid ; if we place it in a pseudoscope—z.e., an instrument which en- 
ables us to view the right picture with the left eye, and the left 
with the right—it stands up like a solid body, just the reverse of 
the effect in the stereoscope. The reason is very simple. Take the 
first case. Here the optical arrangement is such that we are en- 
abled to view the right picture with the right, and the left with the 
left eye. Now, when we direct our attention to the point aq’, 
we have to give a certain convergence to our eyes to make that 
point a as seen by the right eye in the right picture coincide with 
the point a’ as seen by the left eye in the left picture. Now, 
suppose we direct our attention to points 6 b'; these are actually 
further asunder on the diagram ; consequently our eyes require 
less convergence, and we at once assume that point to be more 
distant. In the second instance, of course, the reverse is just the 
