

HERSCHEL ON COLOURED RINGS. ]£5 



" upon the pasteboard, in a quantity sufficient to make them 

 " visible*." 



Mr. Brougham, after having considered what the two au- of Brougham, 

 thors I have mentioned had done, says, "that upon the whole 

 " there appears every reason to believe, that the rings are 

 " formed by the first surface out of the light, which, after 

 '* reflection from the second surface, is scattered, and passes 

 " on to the chart f." 



My own experiment is as follows. I placed a highly po- of the author, 

 lished 7 feet mirror, but of metal instead of glass, that I 

 might not have two surfaces, at the distance of 14 feet from 

 a white screen , and through a hole in the middle of it one 

 tenth of an inch in diameter I admitted a beam of the sun 

 into my dark room, directed so as to fall perpendicularly on, 

 the mirror. In this arrangement the whole screen remained 

 perfectly free from light, because the focus of all the rays, 

 which came to the mirror, was by reflection thrown back ' 

 into the hole, through which they entered. When all was 

 duly prepared, I made an assistant strew some hair-powder 

 with a puff into the beam of light, while I kept my attention 

 fixed upon the screen. As soon as the hair-powder reached 

 the beam of light, the screen was suddenly covered with 

 the most beautiful arrangement of concentric circles, dis- 

 playing all the brilliant colours of the rainbow. A great 

 variety in the size of the rings was obtained by making the 

 assistant strew, the powder into the beam at a greater dis- 

 tance from the mirror; for the rings contract by an increase 

 of the distance, and dilate on a nearer approach of the pow- 

 der. 



This experiment is so simple, and points out the general 

 causes of the rings, which are here produced, in so plain a 

 manner, that we may confidently say they arise from the 

 flection of the rays of light on the particles of the floating 

 powder, modified by the curvature of the reflecting surface 

 of the mirror. 



Here we have no interposed plate of glass of a given 

 thickness between one surface and another, that might pro- 



* Priestley's History, &c. on the Colours of thin Plates, p. 515. 

 , + Phil. Trans, for 1796, p. 216. 



O 2 ducc 



