94 W. B. Rogers on Binocular Vision. 
are converged to an accurate focus on the sentient surface of — 
each eye at the inner extremity of the optical axis. t that this 
may be so, the eyes must be adapted in refractive power to the — 
divergency of the pencils or in other words adjusted to the dis- 
tance of P and Q the actual sources of the rays, although from 
the convergence of the axes to R we are led refer the combined © 
image of these points to that position in s : 
In these conditions therefore it appears that the act of vision is 
attended by a peculiar and unusual relation of the optical adjust- 
ments. For while in the ordinary mode of viewing an object 
with the two eyes we adjust both the refractive power and the 
~ axial convergence to the same gpa viz, that of the particular 
Int -o which or the mom ‘ 
ta a a. of the drawings at the same time that we 
axes to the point behind or in front of them in which — 
figu This capacity of the eyes to de — 
Wheatstone in conne hy vith a peculiar class of experiments 
with his stereoscope (Phil. Mag., 4 series,.vo , but it has not — 
received from writers on physiological opties the Peeniice to 
which, from its relations to constrained binocular vision in gell- 
eral, iB would seem to be entitled. 
The mode of union of the two adjustments of refractive pow 
and axial convergence which has been habitual from infancy cat — 
ciation of roped ee that I would ascribe the ‘pect 
sense of constraint which at first accompanies the above met- 
tioned experiments in binocular vision. 
This feeling of constraint so embarrassing to an inexperienced 
observer ceases after we have learned by a little practice to ee 
rate the two optical adjustments as above mentioned. We thet 
find no difficulty in combining the drawings into a precise 
clear resultant, and fixing the attention upon it as steadily anu" 
an actual body i in the same position. 
