THE ARCHIPELAGO OF BREHAT. 131 



be necessary to augment the intensity and pene- 

 trating power of the light employed. The problem 

 became from that time a more complex one. It was 

 necessary to augment the intensity of the light, and 

 still more necessary to collect and bring horizontally 

 back to the sea the rays which, escaping in all 

 directions, were lost in space, either by falling at the 

 foot of the light-house, or by uselessly diffusing them- 

 selves over the neighbouring land. 



Many attempts have been made to effect this 

 double object. The substitution, by Argand, of 

 lamps with a double current of air was the first 

 step in advance. An Englishman, named Hutchin- 

 son, towards the close of the eighteenth century, 

 first conceived the idea of placing behind these lamps 

 a metallic mirror, which projected forward a part 

 of the scattered rays. A Frenchman, the Chevalier 

 de Borda, carried this mode of lighting to the highest 

 degree of perfection, by employing as a reflector a 

 parabolic mirror which owes to the particular cur- 

 vature of its walls, the property of transmitting, in 

 the same direction, all the rays emanating from a 

 luminous centre placed in its focus ; and of thus 

 projecting forwards a sort of cylinder, composed 

 of all the rays emitted from this centre.* But this 

 advantage involved in itself a very serious incon- 

 venience, for the cylinder of light presented very 



* The invention of the parabolic mirror is not the only service 

 that Borda has rendered to navigation. He was the founder of 

 the schools of naval architecture, and he has left several works, 

 amongst others, a Voyage en diverses parties de VEurope et en 

 Amerique. Borda died in 1799. 



K 2 



