222 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



light visible through a glass, at a distance of forty leagues. Never- 

 theless, the loss of light owing to the reflection or absorption of the 

 rays at the surface of the metal is at least half the incident rays. 

 Moreover, the polished surface of the mirrors is rapidly deteriorated 

 by the action of the saline vapours " contained in the air in close 

 proximity to the sea. These inconveniences have caused the catoptric 

 lighthouses to be abandoned, at any rate in first-class lighthouses. 

 In France they are only used for lighting narrow channels, or in 

 addition to a light in a certain direction where the range of the latter 

 is insufficient. 



But this abandonment was only possible after the invention of the 

 lenticular apparatus, where refraction is totally or partially substi- 

 tuted for reflection, for the projection of the light ; these are called 

 dioptric lighthouses. This invention is due to the illustrious Fresnel, 

 and only dates from the year 1822. 



II. KEFRACTING OR DIOPTRIC LIGHTHOUSES. FRESNEL'S LENSES. 



We have already seen, in speaking of burning glasses, that 

 Buffon thought of constructing lenses formed of concentric portions 

 of lenses of large aperture, thus lessening the thickness of the glass, 

 and consequently the quantity of the heat-rays absorbed in their passage 

 through the refracting medium. These echelon lenses have, however, 

 not been made on a large scale on account of the difficulties in the 

 melting, cutting and polishing of large masses of glass. 



Fresnel who was associated with Arago in the commission nomi- 

 nated in 1819 for the improvement of lighthouses, had the same idea 

 as Buffon, but he greatly improved upon it, and rendered it practicable. 

 In the first place, he made possible and practicable the construction 

 of echelon lenses of large aperture, by forming them of several pieces 

 which can be worked separately, and by subsequently joining all the 

 parts of the lens with a cement of isinglass which causes them to 

 adhere firmly by their edges. In the second place, he profited by 

 this mode of manufacture to improve the form of the refracting 

 surfaces to a degree of which Buffon never dreamt. After saying that 

 if our great naturalist had never made an echelon lens of three feet in 

 diameter it was because the idea never struck him of employing several 



