THE ARCHIPELAGO OF BREHAT. 137 



illuminating power is concentrated by lenses in 

 either eight or sixteen directions only, in a fixed 

 light it spreads over all points of the circle, dimi- 

 nishing in intensity in proportion to the greater 

 extent of surface over which it is diffused. 



A lamp which is placed in the centre of a fixed or 

 moveable apparatus emits its rays in all directions, 

 and consequently a large number pass above and 

 below the lenses. To avoid this loss, Fresnel pro- 

 posed to re-collect these rays upon reflecting prisms, 

 which have the property of destroying only a small 

 quantity of the light which traverses them. This 

 idea was actually applied to lighthouses of small 

 dimensions, but it had hitherto been regarded as im- 

 possible to construct curved prisms of a size suitable 

 for buildings on a large scale. These prisms were 

 replaced by a system of concave mirrors covered 

 with tinfoil, and arranged in horizontal zones above 

 and beloAV the apparatus. Now we have already 

 seen that half the light is destroyed by this mode of 

 reflection ; hence it was most desirable that the 

 prismatic rings should be formed on a large scale. 

 The problem which had hitherto been regarded 

 as insoluble was successfully solved by a Parisian 

 artisan, M. Fran9ois who, in 1844, presented to the 

 Academy of Sciences one of the eight staves, which 

 when combined form the reflecting cupola, as ex- 

 emplified in the lighthouse of Skerryvore in Scot- 

 land, the construction of which is in exact accord- 

 ance with Fresnel's plans. The importance of this 

 improvement will be best understood when we con- 

 sider that the light reflected by the tinned mirrors is 



