162 J. LeConte on Binocular Vision. 
tary act of sight. In the human eye, therefore, and also in 
that of the highest animals, there are three conditions of the 
= axes: Ist, convergence, when we look at a near object; 
n 
in sleep, in drunkenness and probably in death. The Ist 
requires a distinct voluntary effort—a eae voluntary con- 
traction of the ocular muscles; in the 2d there is no voluntary 
contraction, but only that tonic contraction chen aie of the 
waking state; in the 3d the relaxation is complete. The Ist 
is the active state of the eye; the 2d the waking passive state ; 
the 3d the absolutely passive state. 
3. In most normal eyes, in the waking passive state, the 
optic axes are eae parallel, and it is impossible to carry 
the relaxation so far as to produce divergence. Hence it is 
at a more distant point—either by too great or too small con- 
vergence; but it is impossible to donb a an infinitely distant 
object, like a star, except in one way, viz: by convergence—by 
crossing the eyes. For the same reason it is impossible for 
most normal eyes, without the use of instruments, to combine 
stereoscopically two similar objects or two similar pictures, 
ond the plane of the object or the pictures, unless the distance 
between identical points of the objects or pictures be, at most, 
not . see the interocular sees Yet I have known, 
at least, one man, a gentleman of rare intelligence and muc 
‘bicuiinich in :aiteiaiie experiments, and whose eyes were to 
all appearance Siegen e normal, in whom the waking passive 
state, as in gazing on vacancy, was one of slight optic divergence 
—in whom therefore the relaxation was more complete than in 
most eyes. This gentleman could double a star by gazing 
vacantly, and as it were beyond it. I thought at first that the 
doubling was the result of optic convergence, but by placing & 
sereen alternately before the one and the other eye and a 
which image disappeared, I completely satisfied myself that 
his eyes, while gazing on vacancy, were really slightly ‘divergent 
instead of parallel. 
I proved this, however, still more satisfactorily in another 
way. I have stated that it is impossible for most persons of 
normal eyes to combine two similar pictures with the naked eyes, 
so as to form a stereoscopic image beyond the plane of the pictures, 
unless the distance between identical points in the pictures be, 
at most, not greater than the interocular distance. vik this 
when the distance between identica ee nts was greater than 
the distance between the centers of his pupils. His ocular 
