348 FOURTH REPORT — 1834. 



coloured rings 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 Duke 

 de Chauhies 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 scattering fine powder in 

 the air before the mirror f ; and M. Pouillet has ascertained that 

 similar rings are formed when the light incident on the mir- 

 ror is simply transmitted through an aperture of any form in 

 an opake screen X. More recently Mr. Whewell and M. Que- 

 telet 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 phenomenon is 

 to be referred to a different class from those last considered. 



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 Duke 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 repasshig of the ray through the refracting sur- 

 face II . The complete investigation, as far as relates to the di- 

 mensions of the successive rings, is given by Sir John Herschel ; 

 and the formula obtained is found to agree precisely with New- 

 ton's measures ^. 



When the interval between two glasses is filled with different 

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

 subdivided state, the portions of light which have traversed them 

 are in a condition to interfere, the interval of retardation de- 

 pending on the difference of the velocities of light in the two 

 media. Accordingly, coloured rings will be seen when a lumi- 

 nous object is viewed through the glasses ; the rings being 

 similar to those usually seen by transmission, but much larger. 



* Mem. Acad. Par. 1755. + Phil. Trans. 1807. 



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



§ Correspondance Mathematiqite, torn. v. p. 6, et torn. vi. p. 1. 

 II " On the Theory of Light and Colours, " Phil, Trans.; and Encycl. Brit,, 

 Art. Chromatics. 



5f Essay on Liyhl, Art. 679, et seq. 



