166 J. LeConte on Binocular Vision. 
In these experiments the size of the diagrams is of little 
importance. I have used them of every size from 5 by 10 
some practice, have succeeded in all except the most difficult 
of them. Their results agreed perfectly with my own, Ina 
fifth case, however, in which great difficulty was experienced 
and the results were uncertain, I was led to believe that the 
eyes in convergence rotated inward. It is not improbable, 
therefore, that normal eyes differ in this respect. ; 
We believe, therefore, that we are justified in the conclusion 
that when the cye is in its primary position and therefore 
assive, the vertical line of demarkation coincides with the 
vertical meridian and the horizontal line of demarkation with 
the horizontal meridian of the eye, and therefore these two 
lines of demarkation are at right angles to each other. But as 
soon as the eyes begin to converge, the oblique muscles (pat 
ticularly the inferior oblique) begin to act, rotating the eye 02 
the optic axis and slightly distorting its form; so that th 
vertical line of demarkation is now not only no longer coimel- 
dent with the vertical meridian but also no longer at right 
angles to the horizontal line of demarkation, Both the rota 
tion and the change in the relation of the two lines of de 
markation increases with the degree of optic convergence. Tt — 
is possible that the frequent action of the muscles distorting 
the globe of the eye may leave some permanent impress upo? 
. the form of the globe, so that even in a passive state the vert 
cal line of demarkation does not coincide perfectly with 
vertical meridian. If so, then to that extent Helmholtz’s 
principle of real and apparent vertical in the primary position 
of the eye would be true. Or to express it differently : We 
have seen that the inclination of the vertical upon the horizontal _ 
ne of demarkation decreases as the point of sight recedes ; at 
1} inches it is 5°, at 2:2 inches it is 14°, at 7°5 inches #8 F 
25', and at 12 inches 20’, It is possible that even when the — 
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