188 Scientific Proceedings, Royal Dublin Society. 
various experiments it occurred to me that much of the difficulty 
in joining the pictures arose from that facility we have for 
altering the convergence of our optic axes in a horizontal direc- 
tion; in fact, that there is no determinate angle of convergence, 
and, consequently, we generally hear persons who cannot join the 
pictures easily, describe them as floating backwards and forwards 
over one another. It appeared to me, therefore, that if instead of 
attempting to displace the pictures horizontally we displaced 
them vertically by some simple optical contrivance, we should 
get rid of much of this difficulty. The result has justified my 
anticipations most fully, and is a stereoscope which I find can be 
made use of by persons who have the greatest difficulty in seeing 
with the ordinary construction. It has the great advantage of 
being practically unlimited in its application. It can be used for 
paper prints or transparencies, any size, up to even views thrown 
on the screen by the magic lantern. It can be made ofan infinite 
variety of forms, and from the principles of its construction it is 
evident that when once adjusted it will be right for all persons, 
no matter what the distance between their eyes may be. 
As to convergence of the axes, the necessary convergence, if the 
pictures be mounted exactly one over the other, is, of course, 
precisely the same as the convergence of the eyes would be 
looking at any object at the same distance without a stereoscope 
at all, and I am strongly of opinion that these are the easiest 
possible conditions for the majority of persons. However, anyone 
can easily try this crucially for himself in this instrument.* 
As to the various forms this stereoscope can be made in their 
name is legion, but I will point out a few :—1. It may be made 
as a pair of prisms or reflectors attached to an ordinary grapho- 
scope, and looked into at a convenient angle somewhat similar to 
that usually adopted by microscopists, as Fig. 1, Plate 11. 2. It 
may be made somewhat of the form of the old optical pillar, the 
photographs being placed horizontally on their backs, and the 
* While there is no doubt that this stereoscope completely mects the difficulty of the 
difference in distance between the eyes of various individuals, it may be argued that it 
does not meet the difficulty of the difference in convergence of the optic axes desirable to 
suit all sights; but the fact remains that people who have never seen properly with the 
ordinary stereoscope have no difficulty in seeing with this. The last instrument, however, 
completely meets the matter of convergence of the axes, as both pictures are (without 
optical means) seen superposed on one another. 
