COLOURS OF THIN PLATES. 77 



will be depicted on it, similar to the transmitted rings of thin 

 plates, the diameters of the rings varying inversely as the square 

 roots of the thicknesses of the mirrors. The Duo de Chaulnes 

 observed that similar phenomena were produced when a metallic 

 mirror was substituted for the glass one, and the rays transmitted 

 through a semi-transparent plate of any kind, or even through a 

 screen of gauze placed at a short distance in front of the mirror.* 

 Sir W. Herschel found that the rings could be produced by scat- 

 tering fine powder in the air before the mirror ;f and M. Pouillet 

 has ascertained that similar rings are formed, when the light inci- 

 dent on the mirror is simply transmitted through an aperture of 

 any form in an opaque screen.^. More recently, Mr. Whewell and 

 M. Quetelet have observed a set of coloured bands, which are 

 formed when the image of a candle is viewed in a plane glass 

 mirror, the candle being held at a short distance in front of the 

 eye, so that the incident and reflected rays may make a small 

 angle. M. Quetelet appears to think, however, that this pheno- 

 menon is to be referred to a different class from those last con- 

 sidered. 



Newton very ingeniously accounted for the colours observed 

 in his experiments, by the fits of easy reflexion and transmission 

 of that [portion of light which is scattered in all directions at the 

 first surface of the glass; and M. Biot has extended the expla- 

 nation to the analogous phenomena observed by the Duo de 

 Chaulnes. Young showed that they could be explained by the 

 interference of the two portions of light which are scattered in the 

 passing and repassing of the ray through the refracting surf ace. || 

 The complete investigation, as far as relates to the dimensions of 

 the successive rings, is given by Sir John Herschel; and the 

 formula obtained is found to agree precisely with Newton's 

 measures.lf 



When the interval between two glasses is filled with different 

 substances, such as water and air, or water and oil, in a finely 



* Mem. Acad. Par. 1755. 

 t Phil. Trans. 1807. 

 J Elemens de Physique, torn. ii. p. 476. 



Correspondance Matketnatiquf, torn. V. p. 6, et torn. vi. p. 1. 

 || " On the Theory of Light and Colours." PM. Trans. ; and Encyel. Brit., Art. 

 CHUOMATICS. 



U Essay on Light , Art. 679, ct seq. 



