268 FRESNEL. 



would perceive no signal. This great evil is overcome 

 by giving, by means of clockwork, an uniform motion of 

 rotation to the reflector. The luminous beam issuing 

 from this mirror is then successively directed to all points 

 of the horizon ; every ship sees the light at one instant 

 appear, and at another disappear ; and if in a great length 

 of coast, as for example from Brest to Bayonne, there do 

 not exist any two light-houses with the same period of 

 rotation, all the signals are, so to speak, individualized. 

 According to the interval which elapses between two 

 appearances or two eclipses of the light, the navigator 

 always knows what point of the coast is in view ; he finds 

 himself no longer liable to mistake the light-house for a 

 planet or star of the first magnitude near to it rising or 

 setting, or even for those accidental fires, kindled on the 

 coast by fishermen, woodcutters, or charcoal burners, 

 fatal mistakes which have often been the cause of deplor- 

 able shipwrecks. 



A transparent lens brings to parallelism all the lumi- 

 nous rays which traverse it, whatever might be their 

 original degree of divergence, provided the point from 

 which the rays diverge be coincident with that point be- 

 longing to the lens which we call its focus. Glass lenses, 

 then, may be substituted for mirrors, and in fact a light- 

 house with lenses has been long ago executed in England 

 under the idea, at first sight very plausible, that it would 

 be much more brilliant than light-houses with reflectors. 

 Yet it was found in practice that mirrors, notwithstand- 

 ing the gross loss of light which they produce at their 

 surface in the act of reflexion, direct to the horizon a 

 more intense beam of light. Lenses were therefore aban- 

 doned. 



The unknown author of this abortive attempt proceeded 



