Double and Inverted Images on the Retince. 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 lastly, 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, i. 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. 



3 r2 



