BUFFON 361 



tion of the sun's rays by means of lenses. There was in 

 his day a very narrow limit to the size of lenses of 

 glass ; even if large lenses could have been cast, they 

 must have been enormously heavy, and would have 

 intercepted much light. Buflfon proposed to replace 

 the solid lenses by stepped lenses {lentilles d echelons), 

 in which the necessary deflection of the rays was to be 

 efiiected by concentric glass rings, ground out of a single 

 piece of glass, but he was unable to get his invention 

 practically tried. Condorcet,^ in 1773, showed that the 

 stepped lens might be built up of separate rings; 

 Fresnel, in 1822, actually constructed such lenses, and 

 used them to condense the beams of light emitted from 

 the Tour de Cordouan. It is well known that modern 

 lighthouses are furnished with powerful lenses, built up 

 either of rings or prisms. 



THE JAEDIN DES PLANTES 

 A Jardin du Eoi was founded in 1626 as a small 

 garden of medicinal plants. The king's physician, Guy 

 de la Brosse, gave up for its use his own house and 

 grounds, which stood just outside the city. For the 

 first hundred years the garden achieved no fame. The 

 physicians who were one after another called upon to 

 manage it were busy with more remunerative employ- 

 ments, and the reputation of the garden had sunk very 

 low when, in 1732, the Academic des Sciences advised 

 that a practical man of science should be put at its 

 head. Du Fay, whose experiments on vitreous and 

 sulphurous or resinous electricity, and on the conduc- 

 tion of electricity through wet string for a distance of 

 over 1200 feet, are still remembered, and whomFonten- 

 elle commemorates as the only man who had ever 



^Eloge de Buffon, Paris edition of 1804, p. 35. 



