OPTICS. 357 



2ndly. " The invention and sure demonstration of another mirror 

 which, receiving the dispersed beams of any material fire, or flame, 

 yieldeth also the former effect, and serveth for the like use." 



Long after this, viz., in 1726, M. Du Fay found that " at 200, 300, 

 and even as far as 600 French feet (about 640 English), the rays of A<c ' 17! 

 the sun received on a plane mirror, one foot square, and thence reflected 

 to a concave one, 17 inches in diameter, consumed combustible bodies 

 in the focus of the latter." 



The success of this interesting experiment, doubtless, stimulated Buffon. 

 Buffon to attempt the production of fire at a distance, after the manner A * * 1 ' 

 of Archimedes, by one reflection only. In the year 1 747, after various 

 trials with combinations of plane mirrors (in number sometimes 

 amounting to 400), placed in a square frame, and brought to bear 

 upon the object by means of screws, he succeeded in melting lead and 

 tin at the distance of about 50 English yards ; and in burning lighter 

 substances, at the distance of 75 yards. This was affected in March 

 and April. With summer heat, and a better apparatus, he expresses 

 a certainty of producing combustion at more than 140 of our yards ; a 

 distance, probably, double that at which Archimedes produced his 

 conflagration. Since the publication of Buffon's results, the scepticism 

 which prevailed in reference to the burning mirror of Archimedes has 

 been rapidly wearing away. The philosopher had not to invent the 

 apparatus for the purpose, but simply to apply what he had previously 

 invented. With regard to the probable construction of Archimedes' 

 apparatus, since this would not be the proper place to enter into detail, 

 we refer to the speculations in Peyrard's edition of Archimedes, torn. Peyrard. 

 ii., pp. 464 508. We must now return to the point at which we A< c * 18 

 commenced this inquiry. 



In the same century with Archimedes, lived Ptolemy Euergetes, Ptolemy 

 celebrated by historians for causing to be placed on the tower of the 

 Pharos at Alexandria, a mirror, which represented accurately all that 

 was done on water or land within its scope ; and by means of which, 

 as some authors relate, an enemy's fleet was seen at the distance of 

 600,000 paces. We do not hold ourselves responsible for the truth 

 of this. Father Abat, whose ' Amusemens Philosophiques'' were pub- 

 lished in 1763, has an ingenious attempt to prove the probable exist- 

 ence of such a mirror at the time specified ; and a copious abridgment 

 of his arguments, by a very able writer, was given in the nineteenth 

 volume of Tilloch's ' Philosophical Magazine.' 



Among the writings which still remain of the celebrated Claudius Ptolemy th 

 Ptolemy, the Alexandrian astronomer, is one on Optics. It is com- 

 prised in five discourses, or books, of which the first is lost ; most part 

 of the remaining four are preserved, and have been carefully examined 

 in the manuscripts, both by Delambre and by Venturi. 



Although, as we have just remarked, the first book is wanting, we 

 are not entirely ignorant of its contents, because each book commences 

 with a recapitulation of what had been taught in the former. Thus 



