Improvements in the Steveoscope. ; 183 
from all idea of strain, the view of the images seen this way in 
the stereoscope is most painful to ordinary uneducated eyes. 
The reason is obvious, when we take up a book and proceed to read, 
having immediately before had our attention fixed on a distant 
object (in which case our optic axes had been parallel), we imme- 
diately, and without effort, see the type distinctly, the axes of our 
eyes have been instinctively adjusted before they had time to 
change from the landscape to the book. It was not that our eyes 
were adjusted after, and in consequence of, our glancing at the 
book ; but the action of our will in causing our eyes to change 
their direction from the landscape to the book simultaneously 
caused a change in the relative direction of their axes. Probably 
the convergence of the axes of our eyes is influenced by two 
causes—firstly, by an association between the necessary focusing 
action cf the eyes, and the muscles which regulate their conver- 
gence (for in almost all cases the focusing and the convergence are 
simultaneous actions) ; and, secondly, by an instinctive knowledge 
of the distance of the object apart from actual ocular demonstra- 
tion. It may be argued that by supplying a sufficient amount 
of magnifying power the pencils entering the eye will be parallel, 
and, therefore, the eyes having to focus on parallel rays should 
assume that convergence suitable to parallel rays, viz., parallelism ; 
but for all that when we take a stereoscope in our hands we have 
an instinctive idea that the object that is in our hands and 
which we are about to examine is not at an infinite distance, and, 
therefore, our eyes naturally and involuntarily converge; it is easy 
to see, then, why, under these circumstances, a view through a 
stereoscope which involves a parallel condition of the axes is not 
the pleasantest. 
No doubt, the old stereoscopes were made with a ridiculously 
large amount of prism power, but I consider that some of the 
modern ones are just as bad the other way, and the consequence is 
that I have found very many persons who have never seen at all 
with astereoscope properly, or if the pictures appeared after great 
exertion to coalesce it was only by a painful effort. 
The reason of this wide diversity of opinion as to the necessary 
convergence probably arises from the fact that it is a personal mat- 
ter, and some persons have much more control over the muscles 
of their eyes than others, this is partly, too, the result of educa- 
