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Oculists occasionally recommend prismatic lenses mounted 
in spectacles to assist persons who suffer from insufficiency 
of the recti interni muscles ; it would he interesting to know 
if those so assisted, have noticed the fallacious appearances 
which the healthy eye can appreciate. The pseudoscopic 
effects are exaggerated by using a prism to each eye, but in 
most persons this produces a painful sensation. 
The explanation of these phenomena, which I offer with 
some hesitation, is based upon the supposition that in 
binocular vision we estimate the distance of an object by 
the degree of convergence of the optic axes. In these 
experiments, when a flat surface appears concave by the 
interposition of the prism : the optic axes are made to 
converge on a point situated behind the real surface, and the 
imagination gradually removes the object to this apparent 
distance. 
When the base of the prism is towards the nose, then the 
flat surface becomes convex, in this case the optic axes cross 
in front of the real surface, and the imagination raises the 
object to that point. A diagram of the convergence of the 
optic axes on an object, before and after the interposition of 
the prism, will show that when the thin edge of the prism is 
turned towards the nose, the effort made to unite the real 
and the refracted image is the same as if the vision was 
directed to a point more distant than the real object. The 
opposite to this takes place when the base of the prism is 
turned towards the nose. It is very possible that the pseudo- 
scopic vision through prisms may have been noticed by others, 
but I have not been able to discover any description of such 
in the w’orks to which I have access. 
