474 



May 4th, 1837. 



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



" On the adaptation of different modes of illuminating Light-houses, 

 as depending on their situations and the object contemplated in 

 their erection." By William Henry Barlow, Esq., in a Letter ad- 

 dressed to Peter Barlow, Esq., F.R.S., and communicated by 

 hirm 



The letter of Mr. W. H. Barlow, addressed to his father, in which 

 the paper is contained, is dated Constantinople, March 14th, 1837, 

 and states that the experiments which he made with the Drummond 

 light, and other means of illuminating Light-houses, and of which 

 he now communicates the results, were undertaken at the request 

 of the Turkish Government, with the view of placing lights at the 

 entrance of the Bosphorus from the Black Sea. The object of his 

 inquiry is to investigate the principles on which the illuminating 

 power, resulting from the employment of reflectors, and of lenses, 

 depends ; and the most advantageous application of that power to 

 the purposes of Light-houses. 



In discussing the relation which exists between the illuminating 

 power and the intensity of an artificial light, he observes that the 

 former is proportional to the quantity of light projected on a given 

 surface at a given distance ; and that the latter is dependent on 

 the quantity of light projected by a given area of the luminous 

 body on a given surface at a given distance. Hence the intensity 

 of a light multiplied into its surface is the measure of the illuminating 

 power, whether the light proceed from one or from several luminous 

 bodies : and the illuminating power is equal to that of a sphere of 

 light, whose intensity and apparent surface are equal to that of the 

 light itself at any given mean distance. 



Within a certain limit of distance, the property of light which 

 produces the strongest impression on the eye, is its intensity j but 

 when the light is so remote that the angle subtended by it at the 

 eye is very minute, as is generally the case in Light-houses, the 

 intensity of the impression made on the retina is proportional 

 only to the illuminating power. The mathematical investigations 

 of the author lead him to the conclusion that all reflectors and 

 lenses of the same diameter have the same illuminating power when 

 illuminated by the same lamp ; and that by diminishing the focal 

 distance, and intercepting more rays, the illuminating power is not 

 increased, but simply the divergence, and consequently the surface 

 or space over which it acts. The author then proceeds to inquire 

 into the comparative utility of lenses and reflectors, and arrives at 

 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 



