210 W. B. Rogers on Binocular Vision. 
seen as a single line are very simple. ‘The optic axes being con- 
verged to one point of it, the images of the point formed in the 
two eyes are referred to the point itself, and as the same converg- 
ence serves a like purpose for the pairs of images corresponding 
to other parts of the line, the union of the two images of the 
line is simultaneous throughout so much of the length as is dis- 
tinctly visible. When the line is of considerable length, a sensi 
ble change of convergence is necessary in tracing it upwards or 
downwards, and in these directions it assumes the character of a 
perspective line, the visual conditions of which will be referred to 
hereafter. Thus the optical signs of a vertical line are uniform 
convergence of the optic axes and a simultaneous and perfect un- 
ion of the two images of the line. It is obvious that the same 
characters belong to yall short lines situated in a plane perpendic- 
ular to 2h plane of the optic axes. But lines oblique to this 
plane, or in other words, perspective lines, are seen under very 
different visual conditions, the consideration of which belongs to 
the following section 
(B) 11. Of the biiseular union of mutually inclined lines. 
If we place on the upper stage of the stereoscope a card on 
which are drawn two lines converging upwards at £ small angle 
(fig. 19), so that ac may be pres ted 
to the left eye and bd to the bake pe 2 t 
if we unite them by converging the axes 
beyond the card, they will form in that 
a perspective resultant correspondin 
with the other in place and length, but Eavibe the reverse a 
tude, that is receding in the upward direction. 
If now we place the card on the lower stage and combine the 
linesae . y cross vision in front of the figure, we obtain 
a perspective resultant which when the component lines converge 
upwards recedes in the upward direction, and when these lines 
are reversed recedes as it extends down wards, 
It is scarcely necessary to remark that in all these experiments 
the betapaiiies attitude of the resultant is a simple consequence 
of the unequal intervals between the corresponding parts of the 
component lines, requiring various degrees of optical convergence 
to effect their union. Thus in the first experiment the upper en 
of the resultant, formed by the union of a and ce, will be situated 
behind the diagram at the point of convergence of the visual lines 
passing through these points from the left and right eye respect 
ively, and the lower end of the resultant will be placed at the 
