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XIII. On the Adaptation of different Modes of Illuminating Lighthouses ; as depending 
on their Situations and the Object contemplated in their Erection. By William 
Henry Barlow, Esq. In a Letter addressed to Peter Barlow, Esq., F.R.S. 8$c., 
and communicated by Him. 
Received April 27, — Read May 4, 1837. 
Constantinople, March 14th, 1837. 
Having made several experiments with the Drummond light, and other means of 
illuminating light-houses, undertaken at the request of the Turkish Government, with 
a view to placing lights at the entrance of the Bosphorus from the Black Sea, I 
have been led to observe some facts regarding the illuminating powers of the lights 
themselves, and the increase obtained by the use of reflectors and lenses, which, I 
trust, may not be found uninteresting. 
On the increase of illuminating power obtained by Lenses and Reflectors. 
Let L in the annexed figure represent a lamp ; m, m, two reflectors, which may be 
so adjusted as to throw the reflected images either in parallel lines on a screen at 
P' and P", or at such an inclination as to unite with that of the light itself at the 
centre point P. Let also s s represent a screen of such imperfect transparency as to 
absorb the same quantity of light in transmission as the mirrors m, m absorb by re- 
flection; then in the first case the three images P', P, P" will have equal surfaces and 
intensities*, and the illuminating power will be three times that of the central lamp ; 
and when by a different adjustment of the mirrors the three images are blended in 
one, then the surface will be equal to that of the central image, but the intensity 
three times greater, so that in either case the illuminating power will be proportional 
to the number of mirrors, or to the surface of those mirrors. If, therefore, we con- 
ceive the whole space between m, m to be filled with mirrors, to reflect the light in 
parallel lines on the screen P', P, P", it is clear that the illuminating power of the 
* We reject here the difference in the length of the trajectory of the direct and reflected light. 
MDCCCXXXVII. 2 F 
