LIGHT. 131 



ception of the sound itself; whereas we cannot 

 conceive visible things without form and place. 



The law according to which the sense of vision 

 is thus affected, appears to be this. By the pro- 

 perties of light, the external scene produces, 

 through the transparent parts of the eye, an 

 image or picture exactly resembling the reality, 

 upon the back part of the retina : and each point 

 which we see is seen in the direction of a line 

 passing from its image on the retina, through the 

 centre of the pupil of the eye.* In this manner 

 we perceive by the eye the situation of every 

 point, at the same time that we perceive its 

 existence ; and by combining the situations of 

 many points, we have forms and outlines of every 

 sort. 



That we should receive from the eye this 

 notice of the position of the object as well as of 

 its other visible qualities, appears to be absolutely 

 necessary for our intercourse with the external 

 world ; and the faculty of doing so is so intimate 

 a part of our constitution that we cannot conceive 

 ourselves divested of it. Yet in order to imagine 

 ourselves destitute of this faculty, we have only 

 to suppose that the eye should receive its im- 

 pressions as the ear does, and should apprehend 

 red and green, bright and dark, without placing 

 them side by side ; as the ear takes in the dif- 



* Or rather through the focal centre of the eye, which is 

 always near the centre of the pupil. 



