Double and Inverted Images on the Retine. 493 
timations given by the sense of Sight are made to harmonize with 
those which result from that of Touch. 
When we reflect on the importance and pre-eminence of that 
sense, by which we are placed in relation almost with the Infinity 
of Space, we should bear in mind at the same time, that the con- 
ditions of that sense are necessarily put in dependence on the 
laws of Light; while the sense of Touch, to which we are in- 
debted for our most accurate knowledge of things on the Earth’s 
surface, is altogether independent of those laws. 
In order that the intimations given by these two senses may 
correspond with one another, it would appear, first, that certainly 
in some, probably in all animals, the structure of the optic nerve 
brings the impressions, which form inverted and reverted images 
on the retina, into the same order on the sensorium, as those 
which might result from the touch of the same objects ; secondly, 
that in those animals which can direct both eyes to one point, the 
partial decussation of the optic nerves, generally, if not univer- 
sally present, enables the images produced by an object on the 
corresponding parts of the retina of the two eyes, to co-operate 
in producing one impression on the sensorium, and one sensation 
in the mind; and Jastly, that the decussation at the pyramidal bodies 
enables those animals to acquire correct information as to objects 
of sight, from impressions made by them simultaneously on both 
optic lobes, 7. e. on both sides of the sensorium, notwithstanding 
that the impression on each side of the sensorium comes from the 
opposite side of the object in view. 
Nothing is farther from my intention than to represent this 
subject as exhausted, or these conclusions as ascertained ; but in 
the present state of our knowledge, I think it may be said, that the 
inquiry has led to a probable solution of two difficulties, long felt 
in Physiology,—the cause of single and correct vision by double 
and inverted images, and the use of the decussation at the pyra- 
mids. And if the theory shall be found to be incorrect, it may 
still be of use, by acting as a stimulus, and to a certain degree as 
a guide, to farther inquiry. 
3R2 
