ON ILLU.MINATION. 0„J 



What has hindered these shades from being more generallyrhe use of 

 employed is probably an opinion, that they must '^^^^-sarily j|j^'^'.'^^;^^h^'JJ^^^°«f^ 

 eccasion a great loss of light. I hope to be able to shew that,.otion that 

 that opinion is not well founded. ^■^'^'' ^^^^'^^ » 



The following simple experiment was made some years ago, 

 with a view to determine nearly the quantity of 'ight which is 

 lost in passing through a roughened glass. 



Two wax candles, of equal size, lighted, and burning with but experimer.t 

 the same degree of intensity, were placed in two vertical ^'-^o^^rheVjdclass 

 liaders of fine glass, pretty thin, six. inches in diameter and six intercepts no 

 inches in heiirht, the one of smooth and the other of roughened "^""^ ^'^^'' ^'^^" 

 glass ; these two cylinders being placed at the same height, on 

 two tables, at the distance of eight feet from each other, in a 

 room where there was no other light than that emitted by the 

 candles, I presented to the two candles, placed in their cylinders, 

 a sheet of white pajier, at the distance of sixteen feet from each 

 of them, and I interposed before the pnper, at the distance of 

 two inches from its surl^ice, a small wooden cylinder in a ver- 

 tical position, which projected two shadows on the paper. 



J was much surprised to find these shadows very nearly ofXhe causs is 

 the same intensity. This result shewed me that the quantity "^^"'''"'^^ » 

 of light lost in passing through a roughened glass is much less 

 than I at first supposed ; but, on reflection, I saW that there 

 was nothing in the result of the experiment which did not ad- 

 mit of an easy explanation. 



Although roughened glass appears opaque, it is by no mennsfor the minute 



so. In the operation of rouohenincj its surface, which from^'^'^'V^\"* 



^ . . rongli ^rbssare 



being smooth becomes furrowed, and broken in every direction, polished. 



it at last presents an uninterrupted collection of asperitlts 

 of every different form. Individually they are almost invislbK'i 

 to the naked eye, on account of their smallness ; their sides 

 are however smooth and shining, as is easy to perceive on eis:- 

 amining them with a microscope, it is evident that the light 

 which fiills upon the smooth surface of one of these little pro- 

 minent points must penetrate the glass with the same ease 

 (when the angle of incidence is the same) as it would T>enetrate 

 the plane surface of a large polished plate of the same sort of 

 glass; and that having passed through the suiface, the rav 

 must pursue its course in the substance of the glass, and pass 

 out on the ether side in the same manner in une case as in the 

 other. 



Voi. XIV.— May, 1806. F Wnen 



