SUCCESSIVE IMPROVEMENTS. 267 



which are horizontal, directed towards the land, have 

 only been produced to be entirely wasted. 



This horizontal beam of rays not only forms a very 

 small part of the total light ; it has also the serious incon- 

 venience of diminishing in intensity as it diverges, and of 

 not extending itself to a distance without being sensibly 

 enfeebled. To destroy this unfortunate loss of light, — to 

 profit by all the light which the lamp emits, — was the 

 twofold problem which remained to be resolved in order 

 to extend the range, and thus the utility, of light-houses. 

 Concave metallic mirrors, called parabolic reflectors, 

 have furnished a satisfactory solution. 



When the lamp is jilaced at the focus of such a mirror, 

 all the rays which emanate from it are brought, by the 

 reflexion they undergo against its sides, into a common 

 direction ; their original divergence is destroyed ; they 

 form, as they issue from the apparatus, a cylinder of light 

 parallel to the axis of the mirror. This beam is trans- 

 mitted to the greatest distances with the same brightness, 

 except that the atmosphere absorbs a small part of it. 



Before proceeding further, let us stop to observe that 

 this construction is not without an inconvenience. We 

 thus indeed easily bring to bear on the horizon of the sea 

 a multitude of rays which would otherwise have been lost 

 on the ground, in space above, or on the side towards the 

 land ; and we overcome the divergence of those rays 

 which would naturally be directed towards the navigator. 

 But the cylinder of reflected rays can have no greater 

 breadth than that of the mirror ; the space which it illu- 

 minates has precisely the same breadth at all distances ; 

 unless indeed we employed many similar mirrors, pointed 

 different ways, and even then the hoi-izon would include 

 many large spaces completely dark, in which the pilot 



