XXXI. — On the Absorption of Light by Coloured Media, and 

 on the Colours of the Prismatic Spectrum exhibited by cer- 

 tain Flames ; with an Account of a ready Mode of deter- 

 mining the absolute dispersive Power of any Medium, by 

 direct experiment. By J. F. W. Herschel, Esq. F. R. S. 

 Lond. & Edin. In a Letter to Dr Brewster. 



( Read November 18. 1822 J 



My dear Sjr, 



1. AS I see by a notice in your Journal, No. 13., which has 

 just reached my hands, that your attention has been occupied 

 by the phenomena of Coloured Flames, and the Absorption of 

 Coloured Light by Diaphanous Bodies ; and that you have 

 been successful in the very valuable discovery of a manageable 

 flame, discharging a homogeneous yellow light of an inva- 

 riable character, I shall perhaps be excused, while the contents 

 of your paper are unknown to me more particularly, for sta- 

 ting some phenomena which I have myself observed, relative 

 to this highly interesting department of physical optics. 



2. In the course of my experiments in 1819, on the scale 

 of tints developed by different crystals in polarised light, ha- 

 ving frequent occasion to measure the diameter of the rings of 



different 

 9 



