475 



serve. With this view, the author divides Light-houses into three 

 classes : the first comprising Beacon or Warning Lights, placed in 

 order to prevent the approach of vessels, and which consequently 

 can never be nearer than three or four miles ; the second being 

 Guiding or Leading Lights, placed to guide a vessel, and therefore 

 admitting of a very near approach ; and the third including those 

 which, according to the respective directions in which they are seen, 

 have both these duties to fulfil. In the first we require great il- 

 luminating power, and a long duration of the brightest period, with 

 a small angle of vertical divergence; in the second, less illumina- 

 ting power, but a larger angle of vertical divergence are requisite, 

 while the duration of the extreme brightness is of minor importance ; 

 and in the third, all these properties, namely, great illuminating 

 power, a long duration of the brightest period, and a large angle of 

 vertical divergence, are necessary. 



May 1 1, 1837. 

 WILLIAM LAWRENCE, Esq., V.P., in the Chair. 



Henry S. Boase, M.D., and William Tierney Clark, Esq., were 

 elected Fellows of the Society. 



A paper was in part read, entitled, " On the connexion between 

 the Phenomena of the absorption of Light and the Colours of thin 

 Plates." By Sir David Brewster, K.H., F.R.S. 



The Society then adjourned over the Whitsun week, to meet again 

 on the 25th instant. 



May 25, 1837. 



FRANCIS BAILY, Esq., V.P. and Treasurer, in the Chair. 



The Rev. William Walton and Richard Westmacott, jun., Esq., 

 were elected Fellows of the Society, 



Sir David Brewster's paper was resumed and concluded. 



The phenomena of the absorption of light by coloured media have 

 been regarded by modern philosophers as inexplicable on the theory 

 of the colours of thin plates., and therefore irreconcileable with the 

 Newtonian hypothesis, that the colours of natural bodies are depend- 

 ent on the same causes as the colours of thin plates. The discovery 

 by Mr. Horner of a peculiar nacreous substance possessing remark- 

 able optical properties, of which the author has already given an ac- 

 count, furnished him with the means of instituting a more accurate 

 comparison between these two classes of phenomena. By a careful 

 and minute analysis of the reflected tints of its first three orders of 

 colours exhibited by a single film of the above-mentioned substance, 

 they were found to consist of that part of the spectrum which gives 

 the predominating colour of the tint mixed with the rays on each 

 side of it. In analysing the transmitted beam, bands of the colours 

 complementary to the former are seen,, with intervening dark bands ; 

 and when the analysis is made with a high magnifying power, the 



