Royal Society. 95 



the inference that the advantage gained by the employment of the 

 former does not arise from their superior perfection as optical in- 

 struments, but from their using the light more economically, in con- 

 sequence of their producing less divergence of the rays, both hori- 

 zontally and vertically, and illuminating a much smaller space in 

 the horizon. Rules are then deduced for the application of lenses 

 and reflectors in Light-houses, according to the particular situations 

 in which they are placed, and the purposes they are intended to 

 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 j 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 II. — 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. — Sir David Brewster's paper was resumed and con- 

 cluded. 



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 three first 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 

 spectrum is observed to be crossed throughout its whole extent with 

 alternate dark and coloured bands, increasing in number and dimi- 

 nishing in magnitude with the thickness of its plate. In the phseno- 

 * See Lond. and Edinb. Phil. Mag., vol. x. p. 201. 



