92 REPORT—1850. 
their relative proportions to produce the exact effect which they required. This 
apparatus he had lent to Prof. Stevelly to show his class; and while doing so he was 
surprised to observe, that while the cards were revolving rapidly, if he suddenly turned 
away his head he caught a distinct view of the individual coloured sectors at the 
instant he was losing sight of them by aside view. A few weeks before this he had 
attended the lectures of Prof. Carlile, of Queen’s College, Belfast, on the anatomy 
of the eye and of the ear; and had then become aware of a circumstance connected 
with the arrangement of the optic nerves and their relation to the retina, which 
seemed to him to afford an explanation of this curious fact. The optic filaments 
which originated in the right side of the brain and crossed over to the left eye, on 
entering that eyeball, only expanded into that part of the retina which spread over 
the portion of the eyeball next the nose ; and the similar portion of the retina of the 
right eye was supplied by optic filaments which sprang from the left side of the 
brain. These nerves, however, were united in their action by commissural fibres, 
which stretched in an arch from the one to the other. The other and larger por- 
tion of the retina of each eye, and that on which the images of objects as usually 
seen were depicted, was formed by filaments which sprang from the brain in each 
case on the side next the eye to which they went ; these, after accompanying the 
optic filaments of the other eye to the place where they crossed the optic nerve going 
to its own eye, turned round with a bend and accompanied the latter in its passage 
into the eyeball. These portions of the retina of the different eyes were also united 
into one nervous action by the commissure of the retina; so that the retina of each 
eye was divided into two portions,—the portion next the nose, and the outer and 
larger portion; and these two portions of each eye were supplied by filaments 
springing from opposite sides of the brain, and not united in their action by any 
commissure or connecting nerve. Now, the consequence of the sudden turn of the 
head was, to throw the image from its usual place on to the portion of the retina 
next the nose, affecting a new and fresh part of the retina for an instant only, for 
the motion of the head instantly interposed the socket of the eye and shut off the 
object. The sectors therefore became distinct at that instant, for a similar reason 
that in the beautiful experiment of Prof. Wheatstone the electric spark showed them 
distinct, viz. the instantaneousness of the impression on a nervous expansion coming 
from the opposite side of the brain, and having an entirely distinct action. Prof. 
Stevelly hazarded a conjecture, that one use of this arrangement might be to arrest 
the attention of the owner of the eye, and direct it from objects in the direction of 
the optic axes to those moving objects on the side from which danger might arise. 
The use of this to man in his less civilized state, as well as to the lower animals, is 
obvious. The accompanying diagram from Mayo, was kindly furnished by Prof. 
Carlile, and will render the above descriptions more perspicuous. 
abde, a'b'd'c', optic tracts; cfop, 
c'f'o'p', optic nerves ; cpp'c'd'd, com- 
missure of optic nerves. 
ας, a'c'f', external filaments of the 
optic tracts and optic nerves, which 
are expanded into the portions of the 
retine ; fy, f’g', most distant from the 
nose. 
kim', k'lm, central filaments of the 
optic tracts and optic nerves, which 
are expanded into the portions of the 
retin, m'n', mn, nearest to the nose. 
bdd'b', internal filaments of the 
optic tracts, which joi those tracts 
together; opp'o', internal filaments 
of the optic nerves conjoining the 
inner portions of the retine, mn, m'n’. 
