Permanent Colours of Opake Bodies. 173 



Having formed pieces of fuch glafies, about 

 two inches thick, I inclofed all their fides with 

 black cloth, except at their farther, and anterior 

 furfaces. Each of thefe pieces of glafs vividly 

 exhibited its colour, when viewed by tranfmitted 

 light : but when the tranfmitted light was inter- 

 cepted, by covering the farther furface, the 

 anterior furface afforded no colour, but appeared 

 Hack. 



When plates of Tranfparent Coloured Glafs, 

 fomewhat thicker than window glafs, are viewed 

 by tranfmitted light, it is well known, that they 

 exhibit their feveral colours. 



I intercepted the light, which was tranfmitted 

 through fuch coloured plates, by fixing a piece 

 of black cloth, contiguous to their farther fur- 

 face. The plates, thus prepared, when placed 

 in fuch a diredion, that they form a right angle 

 with the window, appear perfeflly black. 



From the phenomena, prefented by fuch. 

 plates, two obfervations may be deduced, (i.) 

 That the Colouring Particles do not refleft any 

 light. (2.) That a medium, fuch as Sir Ifaac 

 Newton has defcribed, is diffufed over both the 

 anterior, and farther furfaces of tlie plates, where- 

 by objedts are reflected equally, and regularly, as 

 by a mirror.* 



• Whenever it is faid that light is refieaed by the furface 

 of any fubftance, it ftiould be underftood, from this expref- 

 fion, that the refleftion is efFefted by the medium, diiFufed 

 over its furface. 



When 



