VOL. LVIII.] PHILOSOPHICAL TRANSACTIONS. 5H- 



that a change of the thickness occasions a change in the colour; difFerently co- 

 loured rays being thus disposed to be transmitted through the plate, and conse- 

 quently rays of different colours being disposed to be reflected at the same place, 

 so as to present the appearance of different colours to the eye. A variation in 

 the density of the plate, he shows, will occasion a variation in the colour; but 

 still a medium of any density would exhibit all the colours according to the thick- 

 ness of the different parts of it. These observations he confirmed by experiments 

 on plates of air, water, and glass. He also mentions the colours which arise on 

 polished steel, by heating it; likewise on bell metal, and some other metalline 

 substances, when melted and poured on the ground, where they may cool in 

 the open air : and he ascribes these colours to the scoriae, or vitrified parts of the 

 metal, which he says most metals when heated or melted, do continually pro- 

 trude, and send out to their surface, covering them in the form of a thin glassy 

 skin. Optics, p. 194. 



This capital discovery, concerning the colours of bodies depending on the 

 thickness of the fine plates which compose their surfaces, of whatever density 

 those plates be (and which may be of such admirable use to explain the colours, 

 and perhaps in due time the constituent parts and internal structure of natural 

 bodies) Dr. P. hit upon a method of illustrating and confirming, by means of 

 electrical explosions. These being received on the surfaces of all the metals, 

 change their colour, to a considerable distance round the spot on which they are 

 discharged, so that the whole space is divided into a number of concentric circu- 

 lar spaces, each exhibiting all the prismatic colours ; and perhaps as vivid as they 

 can be made in any method whatever. 



It was not by any reasoning a priori, but by a mere accident, that he first dis-' 

 covered these colours. Having occasion to take a great number of explosions, 

 in order to ascertain their lateral force ; he observed that a plate of brass, on 

 which they were received, was not only melted, and marked with a circle, by a 

 fusion round the central spot, but likewise tinged, beyond this circular spot, with 

 a green colour, which he could not easily wipe out with his finger. Struck with 

 this new appearance, he replaced the apparatus, and continued the explosions ; 

 till, by degrees, he perceived a circle of red beyond the fainter colours ; and, 

 examining the whole with a microscope, he plainly distinguished all the prismatic 

 colours, in the order of the rainbow. The diameter of the red, in this instance, 



tioti to succeed Dr. Price ia a congregation at Hackney gave him a temporary residence ; till, in 

 1794-, he sailed for North America, where he settled at the town of Northumberland, in the state of 

 Pennsylvania, for the remainder of his life; of which several particular and ample accounts have been 

 printed; viz. in his Life, by J. Corry, 1804; the same, by his son, and Cooper and Christie; also 

 in the Monthly Magazine, vol. 17; the Monthly Review, vol. 46"; the Phiios. Magazine, vol. 22 j 

 and the Edinburgh Review, vol. 9. N° 17, &c. 



