324 W. B. Rogers on Binocular Vision. 
axes are for the moment directed and “the unity of the percep- 
tion is obtained by the rapid survey which the eye takes of every 
part of th@ object,” (Phil. Mag., May and June, 1844). On this 
theory the a of binocular vision of a perspective line 
wenid be as follo 
«. The Pieeiion ‘of a perspective resultant line in the stere- 
eae would require the optic axes to be successively directed 
so as to unite every pair of corresponding points of the two com- 
ei lines of the diagram, or which amounts to the same thing, 
they should be successively con¥erged to every point of the per- 
patkive resultant. 
. This resultant would of necessity be perneved as a single 
perspective line the locus of all the resultant points 
3. In the case of two intersecting lines appearing instead of 
this Rents these lines should neither of them have a perepece 
live position. 
But ‘nea conditions differ in important features from the phe- 
nomena of the perspective resultant before deseribed. In the first 
place the perspectiveness of the resultant, although not perceived 
when the axes are steadily fixed at any one convergence, ap- 
pears as soon as the varying convergence has united contiguous 
points of the component lines, and it affects the whole extent of 
the resultant, although the observer may have restricted the suc- 
cessive union to only a small fraction of the length of those lines. 
In the second place the perceived resultant instead of being a 
single line, is in most cases composed of two intersecting per- 
spective lines varying their point of intersection and their degree 
of relief. "The same conditions, as we have seen, apply to a phys- 
ical line held in the position of the Gener: resultant. 
It would then appear that the perception of a perspective re- 
sultant line or a physical line in the same altitude, does not re- 
quire the successive convergence of the axes to every poilit. 
The direction and amount of the relief may be sufficiently sug- 
gested by the view of afew contiguous points. he presence of 
two apparently or nearly coincident images would. not of itself 
convey the idea of one or of two per spective lines, but if, along 
with this, we have even through a short distance the impression 
of an ae or diminishing convergence and a change in the 
united direction of the axes, the optical signs of perspectiveness 
may be sonedad as sufficiently complete to give us the percep- 
tion either of two intersecting perspective lines of variable relief 
or ea of a single line with the maximum relief. 
n the line is of considerable length, we assure ourselves of 
its stralgiitneen and the degree of its relief by directing the eyes 
successively to all parts of. it, but when it is short the images in 
the two eyes are distinct enough throughout to make a perception 
of the whole line definite and clear, without more than a very 
