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 i)urely historical point of view, the re- 

 positories of industi'ial 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 



