*94 Natural History, 



that refraction did not cease entirely but at the zenithr 

 To conclude : Ptolemy contento himself with vaguely point- 

 ing out in what way wc may determine the quantity of re- 

 fraction ; but he gives no part of it. We were hitherto 

 far from supposing that Ptolemy entertained so expansive 

 and accurate ideas, more especially as there is not a single 

 passage in the '.yhole Almagistus which speaks of refraction, 

 and that it was by a false interpretation that in two places 

 a vague idea was supposed to have been conveyed of their 

 principal effects. 



But it is still more curious, that Ptolemy knew as well as 

 the moderns the refraction which the light undergoes in 

 passing from t-^ie air into water or into glass, and gives 

 tables for all the angles of incidence fron 10 to 100°. He 

 even points out the method of constructing these tables 

 from observation ; and he sees that the angles broken off 

 do not decrease in the same ratio with the augles of inci- 

 dence. Ke had no idea of comparing the sines or the 

 chords of the double arcs ; but from these tables we may 

 easily deduce the relation of the sines ; and what proves 

 that these observations were accurately made, is, that M. 

 Delambre has determined that they differ very little from 

 those which v/«re found by Newton. 



The calculations of Ptolemy are 4 : 3,05656 and 

 3 r 2,0008; those of Newton are 4 : 299432 and 3 : 1,93408; 

 this small diTerence may arise from Nevvton'^s using rain 

 water in his expcrimenls ; whereas Ptolemy may have used 

 common water; and perhaps also the common glass used 

 by Newton had not the same density with the purest glas-^ 

 of the ancients. 



[To be continued.] 



LXX. Intelligence and Miscellaneous Articles. 



TO THE LOVEKS OF NATURAL HISTORY, AND THE GENE- 

 ROUS ANJ) HtjMANE IN GENERAL. 



In a Note at page 82 of the 39th volume of the Philoso- 

 phical Magazine, the Editor took the liberty of requesting 

 the attention of the affluent among his readers, to the di- 

 stressed situation of the ailing widow with six orphan 

 children, and the aged mother, of the late very ingenious 

 Mr.WiLLiAM Martin, F.L.S. Geol. Soc. of London, &c. 

 of Macclesfield in Cheshire, author of *' Outlines of the 

 Method of Extraneous Fossils" (an elementary ^and general 

 work on this interesting subject) ; of " Petrificata Derbi' 

 ensiOf or, figures and Descriptions of Petrifactions collect- 

 ed 



