262 optics. . 



parts of bodies, it would be scattered as much by 

 the most polished substances as by the roughest: 

 and that, therefore, the reflection of a ray of light 

 is effected not by a single point of the reflecting 

 body, but by some power of the whole body evenly 

 diffused all over its surface, and by which it acts 

 on a ray without immediate contact. 



No surface has the reflecting property so pow- 

 erful as to reflect the whole of the light that falls 

 upon it, a part being always absorbed; although 

 there is a great difference in the quantity of the 

 light reflected by different bodies. Metals reflect 

 more light than any other substances. The re- 

 flection of the rays of light from the surfaces of 

 bodies is the means by which they become visible ; 

 and the disposition of bodies to reflect particular 

 coloured rays is the reason why they appear of 

 that colour. 



Every point in an object reflects rays in all di- 

 rections; and, consequently, a certain number from 

 each point enters the pupil, and is converged upon 

 the retina, forming there the images of all the 

 points of the object: hence it is that we see the 

 whole of the surfaces of the bodies presented to us. 



When the rays of light fall upon a rough sur- 

 face, they are reflected very irregularly, and 

 scattered in all directions ; but when they fall upon 

 polished surfaces, they are reflected with more 

 regularity. Such a surface, when highly polished, 

 is called a mirror, or speculum. 



Plane mirrors are those whose surfaces are 

 perfect planes, and whose section is a straight line; 

 such are vulgarly called looking-glasses. 



Convex mirrors are those whose middle parts are 

 more prominent than their extremities or edges, 

 and whose sections are curves, which may be either 



