326 Applicability of the Electric Light to Lightl 



wuses. 



quake, and it remained where it fell until the close of the 

 seventh century of the Christian era, when its materials were 

 sold to a merchant of Edessa for a sum equivalent to thirty-six 

 thousand pounds of our money. The Tour de Cordouan, 

 at the mouth of the Garonne, is one of the most ancient of the 

 modern lighthouses ; a fire of wood was first used in it, then 

 one of coal, and afterwards lamps and reflectors. To it was" 

 first practically applied the dioptric apparatus, which has 

 brought lighthouse illumination to such a state of perfection. 

 Until a comparatively recent period, grates with burning coals 

 continued to be employed to warn the mariner. One of these 

 is still to be seen at Lucerne in Switzerland, in the very place 

 where it was long exhibited for the purpose of guiding the 

 boatmen on the lake at night ; and to it is most probably due 

 the name of the interesting old town, where it still attracts the 

 attention of the traveller. 



The ingenuity of modern times was not satisfied with the 

 contrivances with which the rude nations of antiquity were 

 content. The clumsy grate, with a wood or charcoal fire, was 

 superseded by the more steady and brilliant light of lamps : and 

 means were devised for directing the light thus obtained, in all 

 its intensity, to those points at which it was required. Reflectors 

 were first used for the purpose ; and as, from their nature, they 

 are easily acted on and deteriorated by the atmosphere, it was 

 often sought to supersede them by lenses, but long in vain. 

 The imperfect figure of the lenses which would be sufficiently 

 large for the purpose, the impossibility of obtaining considerable 

 masses of glass free from serious imperfections, the large quan- 

 tity of light absorbed by such great thicknesses even of the 

 most transparent material, all conspired to render the numerous 

 experiments made on the subject unsuccessful ; until the perse- 

 vering ingenuity of scientific men eliminated every difficulty, 

 and at length produced the magnificent dioptric apparatus now 

 in use. 



Buffon endeavoured to diminish the absorption of light 

 which occurs with large lenses, by cutting away their super- 

 fluous portions, and causing them to consist of a number of 

 concentric zones or rings ; but the impossibility of properly 

 polishing the complicated surface, caused at one side of the lens 

 by the different zones, also the liability to irregularity of curve, 

 and the certainty of flaws in the large masses of glass required, 

 rendered the expedient, however ingenious, inapplicable in 

 practice. 



But all these obstacles to complete success were overcome 

 by the sagacity of Sir David Brewster, who, through a long 

 and valuable life, has done so much for optical science. He 

 built up the lens of separate rings, and the rings even of sepa- 



