1 76 Management of Light in Illumination. 



it moves, the ray will be refracted or its direction will 

 be changed. It will appear to be drawn towards the 

 glass before it arrives at its surface ; and its motion in 

 the glass, after it has penetrated through its surface, 

 although it will still be in a straight line, will not be in 

 the same direction in which the ray moved before it 

 approached the glass ; and the same change of direc- 

 tion will again take place, in passing out of the glass 

 into the air, if the surface of the glass where it makes 

 its exit should happen not to be perpendicular to the 

 direction in which the ray moves in the glass during its 

 passage through it. 



Hence we learn that the direction of a ray of light 

 which has passed through a glass, or any other trans- 

 parent substance, will depend not only on its original 

 direction, but also on the refractions it has experienced 

 in entering it and in passing out of it ; and, as these 

 refractions depend on the angles of inclination which 

 the surface of the glass present to the ray, when the 

 surface of the glass is so broken up by grinding as to 

 present an infinite number of small broken surfaces 

 inclined in all directions, the. light which passes 

 through it must necessarily be dispersed. 



Every visible point of the surface of the glass, from 

 which the light escapes, will appear to send off rays in 

 all directions, and this is what gives to the glass the ap- 

 pearance of being luminous ; and it may indeed be said 

 to be luminous without any impropriety of language. 



In the memoir which I presented to the French 

 National Institute, on the 24th March, 1806, on the 

 subject of lamps, I made an observation relative to the 

 usefulness of ground glass for windows, which I shall 

 take the liberty to repeat here. 



