360 Proceedings of the Poyal Society 
that the true optic or visual nerve-fibres from the retinae cross at 
the optic commissure, that they are continued through the optic 
tracts, and sweep inwards to the corpus quadriyeminum; that those 
from the left eye enter that cerebral lobe at the right side, and 
spread across and forward in it in the form of an inverted cone ; 
while the nerve-fibres of the right eye enter the same lobe at 
the left side, and spread in a like manner across it from left to 
right. The fibres from each eye thus cross each other in this 
lobe, which, from being an important central ganglion, and most inti- 
mately connected with the fibres from the optic nerves, the writer 
suggests as the probable sensorium in vision. The effect of this 
simple arrangement is, that the corresponding nerve-fibres from each 
retina are brought into juxtaposition, fibre to fibre ; and in natural 
vision the sensorium thus becomes the organ in which the nervous 
impulses which come from the two eyes are united and grouped in 
the form they occupy on the retinas. 
When, then, in the experiment before-mentioned we advance 
the two strips of card-board but a short way at each side of the 
stereoscope, their images are found on the inner parts of each 
retina, and the ends of the strips are seen as two separate objects, 
because their images are thrown on non-identical portions of the 
retinae, and different parts of the sensorium are accordingly im- 
pressed. When, again, the strips are advanced a little further, 
till the images begin to cross the centre of the retina of each eye, 
the spectator immediately sees the ends to overlap, and at the same 
time to acquire additional brightness. This evidently arises from 
the corresponding parts of each retina being impressed, and the two 
similar impulses being transmitted to that portion of the sensorium 
with which these parts of the retinae are in connection, — each 
nerve-fibre from the one eye bringingits impulse into juxtaposition 
with the corresponding impulse from the other eye. And thus we 
account at once, for the increased brightness, and the apparent 
superposition of the images of external objects. A diagram at a 
glance shows how these are the necessary results of the arrange- 
ment of the nerve-fibres which we have suggested. 
That the nerve-fibres coming from each eye are not united or 
fused in the sensorium, but merely brought into juxtaposition, is a 
fact also proved by the following experiment with coloured strips. 
