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S 
X. An Account of Rings miffing of all the 
Prifmatic Colours , made by Ele&rical Ex- 
plofions on the Surface cf Pieces of Metal , 
by Jofeph Prieftley, LL. D. F. R. S. 
Read March io,ir T was a difcovery of Sir Ifaac Newton, 
I that the colours of bodies depend upon 
the thicknefs of the fine plates which compote their 
furfaces. He has fhown that a change of the thicknefs 
occafions a change in the colour; differently coloured 
rays being thereby dilpofed to be transmitted through 
the plate, and confequently rays of different colours 
being difpofed to be reflected at the fame place, lb as 
to prefent the appearance of different colours to the 
eye. 
A variation in the denfity of the plate, he fhows, 
will occafion a variation in the colour ; but dill a 
medium of any denfity would exhibit all the colours 
according to the thicknefs of the different parts of it. 
Thefe obfervations he confirmed by experiments on 
plates of air, water, and glafs. He alfo mentions the 
colours which arife on polifhed dee!, by heating it; 
as likewife on bell-metal, and fome other metalline 
fubdances, when melted and poured on the ground, 
where they may cool in the open air : and he aferibes 
thefe colours to the fcoria , or vitrified parts of the 
metal, which, he fays, mod metals, when heated, or 
melted, do continually protrude, and fend out to their 
furface, covering them in the form of a thin glaffv 
fkin. Optics , pag. 194. 
This capital difcovcry, concerning, the colours of 
bodies depending upon the thicknefs of the fine 
plates 
