in connexion with Vision. 241 



that it had, at the same time, the sensation of the variations of 

 light which distinguish that object from others placed around 

 it. The usual expression of Dr. Brewster, that the image art 

 the retina is ' seen along' a certain line, implies this :-for these 

 two words signify that the retina, besides discovering that the 

 object is red, blue, or yellow, determines that it is placed to the 

 right or to the left, or above or below, and also the exact line 

 or degree in which it is so placed. 



The main error, which has misled the numerous writers who 

 have treated of this question, and which has created the degree 

 of puzzle that seems always to have been attached to it, may 

 be traced to this that they have invariably sought to solve the 

 problem by a reference to the functions of the optic nerve 

 alone ; they have looked upon the globe of the eye and this 

 nerve as constituting the entire instrument of vision ; without 

 taking into consideration the apparatus of muscles, and their 

 nerves, which, under the guidance of the will, move and direct 

 the eyeball from point to point. It can only have been from 

 taking this partial view of the organ of vision that they could 

 attribute to the optic nerve such complicated and inconsistent 

 functions as those which have been bestowed upon it. 



The explanation of the problem Why an inverted image 

 should give the idea of an erect object, which has been adopted 

 by Dr. Brewster, is founded upon the law which has beeri 

 called the e law of visible direction.' This supposed law, as the 

 words themselves imply, includes the opinion that seeing and the 

 power of distinguishing the direction of objects are possessed 

 by the retina together ; and the following passage, taken from 

 the * Treatise,' will serve to exhibit the nature of the proofs 

 Upon which it rests. 



* On the Law of Visible Direction. When a ray of light 

 * falls upon the retina, and gives us vision of the point of an 

 1 object from which it proceeds, it becomes an interesting ques- 

 ' tion to determine in what direction the object will be seen, 

 4 reckoning from the point where it falls upon the retina. In 

 1 fig. 142, let Fbe a point of the retina, on which the image of 

 4 a point of a distant object is formed by means of the crystal- 

 1 line lens, supposed to be at,L' L. Now the rays which formed 



