XV. On a New Species of Coloured Fringes, produced by the 

 Reflexion of Light between Two Plates of parallel 

 Glass of equal thickness. By David Brewster, LL. D. 

 F.R. S. Edin. & F. A. S. E. 



(Read February 20. 1815 J 



DURING a series of experiments in which I was lately en- 

 gaged, for the purpose of determining the law of the 

 polarisation of light, by successive reflexions from plates of 

 parallel glass, I observed that all the images of the luminous 

 body which were formed by more than one reflexion, were 

 crossed by parallel fringes of coloured light, when the two 

 plates had a small inclination to each other ; and that these 

 fringes suffered considerable changes, by varying the position 

 of the plate with regard to the incident ray. 



These coloured fringes seemed at first to have the same ori- 

 gin as those of thick plates, which were discovered by New- 

 ton, and afterwards examined by the Duke de Chaulnes, Mr 

 Brougham, and Mr Jordan ; and I considered the second 

 plate of glass as performing the part of the quicksilver in New- 

 ton's glass mirror, or of the metallic speculum in the experi- 

 ments of the Duke De Chaulnes, and Mr Brougham. A 

 more attentive examination, however, convinced me that this 



was 



