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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 
