for the purposes of Illumination in Lighthouses. 35 



perform in its focus all the operations of chemistry, as conve- 

 niently as could be done in a furnace *." 



There can be no doubt that the lens thus described by BUF- 

 FON, would have produced the effect which he ascribes to it, had 

 it been possible to execute it ; but though he invented it twenty- 

 five years before he described it, though all the philosophers to 

 whom he mentioned it anxiously desired to see it made, and 

 though sixty years have elapsed since the publication of his 

 work, such a lens has neither been attempted nor executed. 

 The fact, indeed, recorded on the authority of M. ROCHON and 

 M. CHARLES, that BUFFON had constructed a lens with steps 

 made of one piece of glass, and only 12 or 15 inches in diameter, 

 may be regarded as a proof that the principle was not practical- 

 ly applicable to lenses of a large size. So visionary, indeed, did 

 the scheme appear to me, when I read BUFFON'S Memoir, of 

 grinding down a solid lens, five, or even three feet diameter, in- 

 to three spherical surfaces on each face, the one falling below the 

 other, that I never hesitated to suppose that he proposed his 

 lens to consist of three separate rings ; and under the influence 

 of this mistake, I drew up my description of BUFFON'S invention. 

 But though the formation of the lens by means of three sepa- 

 rate rings, would remove in a great measure the difficulty of 

 grinding and polishing the successively descending surfaces, yet, 

 even with this improvement, the scheme is just as visionary as 

 before, since the difficulty and expence of casting, grinding, and 

 polishing a ring of glass, five or even three feet diameter, is as 

 great as to execute a solid lens of the same size. 



But, however this may be, the lens actually proposed by BUF- 

 FON, ingenious as it is, must be ranked among those visionary 

 contrivances which never find a practical application. 



* Supplement a FHistoire Naturelle, torn. ii. 12mo Paris 1774. 



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