OF THE SENSE OF SIGHT. 115 



rays of light into the interior of the eyeball ; they change 

 the direction of the rays of light. The eye is but a kind of 

 camera obscura, the image of objects being as it were painted 

 on the retina ; this image we see, and not the object itself. 



To understand this part of the history of vision, it is only 

 necessary to refer briefly to some of the laws of optics. 



Light travels in straight 

 diverging lines. When they 

 fall perpendicularly on the sur- 

 face of a transparent body, they 

 traverse it without anj r change 

 in their direction; but falling 

 obliquely, they are always 

 affected more or less in their 

 direction. If they are passing 

 from a rarer into a denser me- 

 dium, as from air into water, 

 they are refracted towards the Fig. 65. 



perpendicular ; the opposite 



happens in passing from a denser into a rarer medium ; they 

 are then refracted from the perpendicular. A straight rod, for 

 example, plunged into water, appears bent at the point of 

 immersion ; and by placing a com in an empty basin (Fig. 

 65 a), so that it shall be invisible to the eye of the observer, 

 it will become visible by merely filling the basin with water 

 (e), for then the rays of light from the ewer which for- 

 merly took the direction of c, and did not reach the eye of 

 the observer, will now pass in the direction of d b, and so 

 the coin will be seen. 



