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SpyXI. On the Catoptrkal and Dioptrical Instruments of 

 the Antients. 



LETTER III,* 



1 . As I have reason to think that my two last letters have 

 not been unacceptable to many of your readers, I should now 

 proceed to inquire (according to my means) into the state 

 of catoptrics and dioptrics among the antients. But here our 

 learned and ingenious friend Mr. Johnston, who in translat- 

 ing, as he has ably done, Becfonann's History of Inventions, 

 has had occasion to pay particular attention to the progress 

 of the arts and sciences, has very opportunely put into my 

 hand a work which will leave me little more trouble than 

 that of translation. It is the Ami/semens Philosophiques of 

 father Abat, printed, it would appear, for the first time, at 

 Amsterdam, and sold at Marseilles in 1763. If certain 

 part's of this learned performance could have been known 

 to Dr. S-, whose book appeared in 1738, he would pro- 

 bably have been induced to alter some parts pf that valuable 

 performance. 



2. It is possible that some may think such an investiga- 

 tion has little other use than to gratify an idle curiosity ; 

 and it must be confessed that such an opinion has received 

 but too much countenance from some men of character in 

 the literary world. Dr. Thomas Burnet f, in particular, 



* The first letter is inserted in the Philosophical Ma^uzine, vol. xviii. 

 p. 53, &o; the second, same volume, p. 24$, <kc; and its conclusion, 

 vol. xix. p. 66", &c. — Part of the note to § 21 of letter ad, should be 

 read thus: Mr. Gibson, watchmaker, at Hampstead, a very ingenious 

 mechanician, whose late brother, at Keiso, wftfc one of the first opticians 

 in Great Britain, and who has himself paid particular attention to per- 

 spective, complains, that he scarcely knows a scientific (especially an 

 astronomical) book, the figures of which are free from gross 'en 01 s in per- 

 spective, where perspective is necessary, or is attempted. Hut he ac- 

 knowledges the itierits of many writers on this branch of science, parti- 

 cularly Dr. B. Taylor, Hamilton, fcJoble, Tho. Maltpn, and H.Clarke; 

 and I hazard lit'lc in adding Gravesande, Murdoch, in the first sect, of 

 his Netutoni Genesis Curvaru/n per Umbuts, and VVo'fms. in his Elemevta 

 Matbeseos Univ(rsa. — \x\ § 19. line 9th from the bottom, for ??ierely tead 

 chiefly. § 26, before Kepler, insert The. great. — in the note'to § 38, \ 

 state the sum which Bacon bid out on experiments, &c. in twenty years, 

 at about 18,333!. of our present money , but the learned Dr. Henry (Hist. 

 of G. Biit. vol. viii. p. 217.. 2d ed.) makes it about 30,000!. — At the 

 end of the next note are meant, of course, practical astronomy and navi- 

 gation in their present improved state. — The writer will be thankful for 

 remarks and corrections, if made with good manners. 



f Arcbeeologia Philosopbica-, lib. I. cap. 8, as quoted in Hcaihcote's 

 Untoria Astronomies, p. 7. 



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