Imperial Institute of France. ^gs 



extent ; and, from the circumstance of their ceasing where 

 the greenstone terminates, thev appear to be occasioned iu 

 some >yay or other by this bed, 



IMPERIAL INSTITUTE OP FRANCS. 



{Continued from p. 3)4.] 

 Optics of Ptolemy. 



It was thought that this work was entirely lost : a few 

 Jins only of it, handed down by Bacon and afterwards 

 copied bv Montucla, were supposed to be ail tliat remained. 



Count Laplace was the first to announce ihat the Im- 

 perial Library possessed the Latin trauslaiion made by 

 Aramiratus Eugenius Siculus. M. Humboldt having read 

 this translation, communicated it to M. Delambre, who 

 made an extract from it too copiou* to be inserted here at 

 Jengih, and which he read to the Class on the 7th of Octo- 

 ber. This work, the first book of which was wanting in 

 the Arahic MS. from which Ammiratus translated, con- 

 tains abundance of obscure rtietaphvsics, physical explana- 

 tions which are little better, an erroneous s\ stem on Vision, 

 which we fii)d more amply detailed in the Optics of Euclid, 

 and in Cleomtdes, some true theorems, but demonstrated in 

 a loo'i and tedious manner. A more careful translation 

 would add absolutely nothing to our present stock ot know- 

 ledge. But the errors even of Ptolemy are not without in- 

 terest to the history of science: an extract from a few of 

 its paces wili satisfy our curiosity in this respect : but not- 

 withstanding these impjerfections, the work in question con- 

 tains two very remarkable articles. 



In the first, Ptolemy gives a very precise and complete 

 account of the effects of astronomical refraction. He po- 

 sitively asserts that these effects are the more considerable 

 the nearer the star is to the horizon ; that the refraction 

 constantly brings the star to the zenith ; that it diminishes 

 in appearance the parallel of the described star, because it 

 generally diminishes its polar distance, except however 

 when the star passes the meridian between the zenith and 

 the pole, because then by drawiug the star to the zenith it 

 removes it from the pole: thus, its parallel in this case 

 ought to increase ; but then the effect is insensible, be- 

 cause ihe star is too c\o<v to the zenith. In this respect, 

 therefore, Ptolemy was further advanced than Tycho Brahe, 

 Kepler, Hevelius, and all the astronomers to the time of 

 Cassini, who was the first among the moderns to ascertain 



that 



