in connexion with Vision. 253 



mity to its lowest ; and it is by this sensation, combined with 

 the sensation communicated to the skin, that we determine the 

 position of the object through the sense of touch. 



The most remarkable circumstance connected with this sub- 

 ject is the minuteness and the precision with which the eye can 

 observe the differences of place or direction in objects. This 

 can only be explained by referring to the extraordinary fine- 

 ness of the sensibility to the different degrees of light which be- 

 longs to the spot of the retina situated in the axis of vision, and to 

 the susceptibility of the muscles to perceive the smallest variations 

 in the position of this spot while engaged in directing it towards 

 objects. Things which, from their minuteness, almost elude our 

 naked sight, can be divided into upper, lower, and lateral parts. 



The conclusions to be derived from this mode of explaining 

 the nature of a perception through the organ of vision, are 

 both curious and highly interesting. We learn that the ideas 

 of objects, which we are in the habit of saying are acquired 

 through the eye, are never the productions of that one organ ; 

 but, as if it were for the purpose of certifying the reality of 

 the things around us, and placing this reality beyond the 

 doubts of philosophers, who may have been bold enough to 

 question it, two distinct senses are called into operation. 

 Thus before we obtain the assurance of the simple fact, that 

 the tree before us has its trunk fixed in the ground, and its 

 leaves in the air, the optic nerve must, in the first place, convey 

 the representation of the tree, that is, its colours, shadows, 

 and outlines, by which it is distinguished from other objects ; 

 while, in the second place, the muscles must cause the eye to 

 traverse all its boundaries, taking points, and marking distances, 

 so as to estimate its height and breadth, figure and position. 

 There are thus not only two senses, but a process of compa- 

 rison, calculation, and judgment, which implies an operation of 

 the mental powers, combined in making this simple perception 

 of a tree complete. 



Without going further into this subject, I may be allowed to 

 remark, that to the medical man it is of much importance to 

 study the mode of operation of the organs of the senses, and 

 particularly of that which has been under our consideration. 

 The questions connected with squinting and disordered actions 

 ,of the muscles of the eye, which are still so little understood, 



