th^fyuse of. coloured concentric Rings, SOS 



considered by some of our most acute experimentalists, I 

 have used it at present, though only in one of the various 

 arrangements, in which I shall have occasion to recur. Uv it 

 hereafter. 



Sir I. Newton placed a concave glass mirror at double its 

 focal length from a chart, and observed that the reflection of 

 a beam of light admitted into a dark room, when thrown 

 upon this mirror, gave *^ four or five concentric irises or 

 rings of colours like rainbows*." He accounts for them 

 by alternate fits of easy reflection and easy transmission ex- 

 erted in their passage through the glass- plate of the con- 

 cave mirror f. 



The duke de Chaulncs concluded from his own experi- 

 ments of the same phaenoraena, " that these coloured rings 

 depended upon the first surface of the mirror, and that 

 the second surface, or that which reflects them after they 

 had passed the first, only served to collect them and throw 

 them upon the pasteboard, in a quantity sufficient to make 

 them visible J.'* 



Mr. Brougham, after having considered what the two 

 authors 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, aj'ter re- 

 flection from the second surface, is scattered, and passes on 

 to the chart §.'' 



My own experiment is as follows, I placed a highly po- 

 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 



♦ Newton's Optics, p. 2G5. f Ibid. p. 577. 



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

 § Philosophical Transactions for 1796, p. SI 6. .. : P^ 



with 



