266 FRESNEL. 
houses were but little remarkable ; the feeble rays which 
proceeded from fires of wood or of coal, lighted in the 
open air on their summits, could never penetrate the 
thick vapours which in all climates obscure the lower 
regions of the atmosphere. : 
Nevertheless, as to the intensity of light, the modern 
light-houses were but little superior to the ancient. The 
first important amelioration which they received, dates 
from the double-current lamp of Argand ; that admirable 
invention which would be much better appreciated, if, 
while our museums include works of the period of the 
decline of art in a purely historical point of view, the re- 
positories of industrial science presented successively to 
public inspection the various means of illumination, so 
dull, so bad, so ill-suited, so nauseous, which were em- 
ployed only fifty years ago, by the side of those elegant 
lamps whose pure and brilliant light rivals that of a sum- 
mer day. 
Four or five Argand lamps united, would give without 
doubt as much light as the large fires which the Romans 
used with so much trouble, on the lofty towers of Alexan- 
dria of Puzzuoli, or of Ravenna; but in combining these 
lamps with reflecting mirrors, their natural effects may 
be prodigiously increased. The principle of this last in- 
vention ought to arrest our attention for an instant, 
because it will enable us rightly to appreciate the value 
of Fresnel’s labour. 
The light of a burning body expands uniformly in all 
directions,—one part falls on the ground, and is lost; 
another portion ascends, and is dissipated in space; the 
sailor whose route we wish to enlighten, profits only by 
those rays which are emitted horizontally, or nearly so, 
from the lamp across the sea; all the rays, even those 
