W. B. Rogers on Binocular Vision. 95 
_ 5. Of the unusual adjustment of refraction and axial con- 
vergence with Brewster’s and Wheatstone’s Stereoscopes. 
The sense of unnatural tension above referred to is not confined 
to observations with forms of Stereoscope like that just described. 
It more or less attends the use of all stereoscopes however perfect 
their construction, or accurate the disposition of the drawings. It 
may indeed be stated asa general proposition that whenever a 
perspective resultant is formed by the binocular vision of plane 
drawings, there are certain parts of the resultant which cannot be 
seen without a separate adjustment of the refractive and the axial 
convergence of the eyes. As this condition attending vision with _ 
ordinary stereoscopes even in the best arrangement of the draw 
ings, appears not to have attracted the notice of writers on the 
subject, the following illustrations of it may prove interesting. 
In fig. 6 are represented the essential parts : 
of the refracting stereoscope of Brewster 
now so familiarly known in this country 
and Europe. P and Q denote correspond- , 
ing points of the drawings which by the use’ me 
of the semi-lenses C and D are to. apited 
into one. When these points ate properly 
adjusted as to their distances from 
D and from each other 
t 
best % ki J esti ; + 
by orditiary tnconstrained vision. In this * 
case the conical pencils proceeding from P 
and Q have their virtual foci coincident at 
¢ m 
BR * * 
ns 
justment this coincidence does not take place. Thus if P and @ 
be moved farther from each other to P and Q, the virtual focus of 
| ata point V beyond R, those for s and ¢ intersect one another at 
a point X nearer than R. 
reosco der 
to unite m and nm we must make the same separate adjustments 
of refraction and axial convergence as when using the upper pits 
and forming the resultant beyond the drawings—and to combine 
_ sand ¢ we must proceed as if using the lower stage and viewing 
the resultant nearer than the drawings. But it is obvious that 
i 
ee 
