IMPROVEMENTS UPON FKESNEI/S SYSTEM. 187 



very slowly, and all that is required to secui-e the steady 

 continuance of motion is that the clock-work should be 

 wound up every day when the lamps are trimmed, and set 

 a-going when the lamps are lighted at night. Of course, 

 as each set of three lamps comes to the front, their com- 

 bined light sends a flash far over the sea. 



A mode of constructing lenses of a size sufficient to be 

 used in light-houses was at last devised by a French phil- 

 osophical engineer named Fresnel; and so great was the 

 success of his system, that it came soon to be almost uni- 

 versally introduced, and has connected his name indissolu- 

 bly with the light-house system all over the world. Very 

 great improvements have been made in his system by oth- 

 er inventors, but they do not displace his name as the orig- 

 inator of the idea out of which they have all proceeded. 

 . It is only an idea of the general principle of Fresnel's in- 

 vention that can be given in a chapter like this. It con- 

 sists essentially in "building up," as it were, a lens for the 

 concentration of the rays, by forming it in separate por- 

 tions, each portion except the central one, and sometimes 

 even that, being in the form of a ring, the surfaces of all 

 the portions being so arranged as to produce the same ef- 

 fect in refracting the rays as if the lens was made in one 

 solid mass. 



To 'understand this clearly, we must consider that the 

 function which it is required of the lens to perform is to 

 draw in the rays somewhat from their natural divergence, 

 since in issuing from the source they would, if left to them- 

 selves, diverge too widely. Now it is the property of a 

 convex lens to produce this effect, as we see exemplified 

 in the case of the sun-glass, so called, which is often used 

 as a toy to concentrate the light and heat of the sun. 



Now, to make a lens of this form, and of the size which 

 would be necessary for a light-house, would require a very 



