3 214 W. B. Rogers on Binocular Vision. 
pair of inclined lines on the lower stage = 
brings them into the same relation to the 
of lines less inclined to one another placed 
on the upper stage. 
From a comparison of the results above 
described we may I think, draw the follow- 
ing conclusions : 
First. The eyes are disposed naturally or 
from habit to incessant slight changes of con- 
vergence which are effected unconsciously 
and with great celerity. Hence the con- 
_ tinuance of any given state of convergence 
implies a constrained state of vision and re-_ 
Wires a special effort of wil 
Second. Great variations of convergence 
require a sensible and even considerable 
time, and are not spontaneous, bas the result 
of ———- effort. 
Third. The successive union of corres- 
ponding points belonging to lines slightly 
nclined to one another, is effected almost in- 
ron gee and without risa effort. 
ich points in lines having a much 
greater Sdelinasion is effected slowly and by 
a + nena exertion. 
Fourth. In the binocular combination of 
two mutually inclined lines no perspective 
effect is produced so long as the convergence 
of the optical axes is kept unchanged. 
, Fifth. € apparent relief or perspect- 
iveness of the resultant presents itself only 
where contiguous corresponding points of 
the two lines are combined in rapid succes- 
sion. Hence it never occurs when the lines 
are greatly inclined to one another, and it al- 
ways shows itself in greater or less extent 
when their inclination is so small as to re- 
quire an inconsiderable change of converg- 
ence 
Siz th. This perspectiveness is seen as well 
when the resultant is obviously com posed of 
two intersecting lines, as when it appears as a single line, and it 
is destroyed whenever by voluntary praia we suspend the 
usnal or spontaneous changes of convergenc 
In referring the perspectiveness of the seadtaa line or lines to 
the rapidly successive union of corresponding points, I would not 
