disperse the 

 rays/ 



2,Q ON ILLUMINATION. 



These small When a pencil of paralcll rays fulls perpendicularly upon a 



surfaces only plate of well-polished glass, they pass through the glass with- 

 out any perceptible change of direction ; but when the pencil 

 falls upon a plate of roughened glass, the rays of which it is 

 composed are dispersed, and the cylindrical pencil is trans- 

 formed into a cone. The ultimate course of each ray depends 

 on the refractions that it has undergone in entering and issuing 

 out of the glass, and these refractions are determined by the 

 angles of incidence, and the respective inclinations of the re- 

 fracting surfaces on each side of the glass at the point where 

 the ray enters and at that where it goes out. 

 A clear glass If the flame of a lamp be placed in the center of a globe of 



globesiinouiKl-|^fjg glass, well polished, the ravs issuing: from it will traverse 

 ing a lamp is ^ r i i i • i "' i • 



scarcely seen ; ^n^ sides or the globe without undergomg any perceptible 



if the glass be change, either in their intensity or their direction : and the 

 roughened it ^ ... , ,. . , , , • . 



emits light name Will be seen so distinctly througlv the globe that this last 



fronieverypart. might even escape observation. But if, instead of a globe of 

 polished glass we employ a globe of roughened glass, in that 

 case, the rays emitted by the flame will be dispersed by the 

 glass in such a manner that each visible point of the surface of 

 the globe will become a radiant cone, and consequently the 

 globe will appear luminous, diffusing light from its surface ia 

 every direction. 

 The light is From this explanation of the phenomena we see that a shade 



and^ver '''^little ^^ ^"^ 3^"^^ roughened, when it is used with a view to disperse 

 lost, and soften the too vivid light of a lamp, does not occasion any 



considerable loss of light. This loss would even be imper- 

 ceptible, or not greater with a. shade of roughened glass than 

 with one of the same kind of glass polished and transparent, 

 notwithstanding the great dispersion of the light, were it not 

 for the reflections which some of the rays suffer before thty 

 quit the shade, 

 y internal re- It is well known that when a ray of light falls upon a po- 

 lished surface of glass (or other substance), at a very small 

 angle of incidence, it is necessarily reflected ; and as the sides' 

 of the asperities of the roughened glass must present themselves 

 to the rays which proceed from the lamp at angles of all pos- 

 sible magnitudes, there must necessarily be some whose incli- 

 nation is sufficient to reflect some of the rays that fall uport 

 thcrn ; and as that raay occjr at both surlaccs of the shade, it 



is 



flections. 



