SIGHT AND THE VISUAL ORGAN. 473 



the eye the yellow spot marking the entrance of the optic nerve. All 

 perception whatever is arrested within the hounds of this spot it is a 

 blind point in our field of view. 



The blind spot is by no means so excessively small. At the dis- 

 tance of four paces, it would cover a man's head in the centre of your 

 field of view; and almost a hundred moons in the sky would find room 

 within its bounds. When Mariotte made this important discovery, it 

 caused so much commotion that the experiment had to be repeated 

 before the King of England in 1688. In the endless variations of the 

 experiment, the remarkable fact only received a new confirmation. 

 For the rest, this discovery had almost proved fatal to the doctrine of 

 perception ; for, as at that time the optic nerve and the retina being 

 considered as essentially the same, one might deduce, a priori, the in- 

 ference that, just at that point of entrance at which all the conducting 

 fibrils converged, a heightened sensibility might be argued. But, now 

 proving to be insensitive, the retina itself could no longer be regarded 

 as the regular conductor of the sensation of light. And this was the 

 conclusion to which Mariotte did arrive, transferring the sensibility 

 to the choroid behind the retina, till at length Bernoulli and Haller 

 again restored it to its rights. 



This apparent enigma is explained by what I told you of the gen- 

 eral relation of light to the visual organ. The part the optic nerve 

 plays is only that of a conductor, while the sensations of the vibrations 

 of the ether, as also of the specific sensory irritation, is committed to 

 the retina, or, more correctly speaking, to its external layer. 



Another question is, Why does the existence of the blind spot 

 usually escape our attention ? The chief reason is that, as the gap is 

 regularly situated in the same spot in the field of view, the idea has 

 learned to fill it up in the most natural manner, and as is suitable for 

 the connection of objects. For instance, I draw the figure of a cross 

 on the board, and fix my eye on it, so as to cause the centre of this 

 figure enclosing the point of intersection to fall on the blind spot ; in 

 this manner I believe indeed that I see a cross, while in reality I only 

 see what lies beyond ; fancy supplementing the rest. The cross is a 

 commonly-known figure, and when any two lines take a perpendicular 

 direction toward each other, they as a rule really do intersect each 

 other. The best proof of this being the case is that, when you obliter- 

 ate all that lies within the district, you still continue to see the cross; 

 and, to make the experiment more elegant, if you place some photo- 

 graph in the empty space, you do not perceive it, you still continue to 

 see only the cross. You have here, then, a conjunction of objective 

 sensory action and subjective influence, apparently with the help of 

 the central extremity of the optic nerve, which is highly significant 

 for the whole doctrine, and which to a certain degree combines what I 

 have been endeavoring to explain to you on both of those branches 

 of the subject. 



