Instruments of the Antients. 1 85 



the other about a palm in diameter, and about 5 brasses 

 in focal distance*. Scarabelli assures us, not only that 

 both these lenses readily inflamed combustible bodies placed 

 in theirybci, but also that each of them could be used with 

 good effect as a telescope, in the open country. 



16. u Further; in searching for what old authors have 

 written on the properties of convex lenses, I have found that 

 John Baptista Porta, more than a century before father 

 Scheiner, and even before the use of telescopes became com- 

 mon, knew that their end misfit be answered by a single 

 lens ; for in his Magia Naturatis* lib. 17. cap. 10, he pro- 

 pounds this problem : * With a lens of glass, to see distant 

 objects as if they were near/ Of this problem he adds an 

 exact solution, although he gives neither the dimensions of 

 the lens nor its focal distance; and does not enumerate the 

 precautions which are necessarv for success, f lis solution 

 is this: ' Posito ocido. 9 &c. Having placed your eye in its' 

 centre, look at the remote object ; for you will view distant 

 things so near, that you will seem, as it were, to touch them 

 with your hand, and to see the clothes, complexions, and 

 countenances of men ; so that you may distinguish vour 

 friends, though at a very great distance/ He knew the im- 

 portance of this discovery ; for in proposing it he observes, 

 that it well deserves to be reflected on : Quod sequitur, says 

 he, longe prcestantius vobis cogit audi prim iphnn affert. 



1 7. H Porta knew farther, that the same thing might be 

 done with a concave mirror : for, in the 1 1th chapter of the 

 same book, when speaking of the mirror of Ptolemy and its 

 effects, he says, that he is wishful to show how it may be 

 executed, so that we may know our friends at the distance 

 of some thousands of paces (or some miles f), and read from 

 afar very small characters. He assures us, moreover, that 

 the thing is extremely easy. It is true that his explication 

 of it is not intelligible ; but it appears that he has done this 

 on purpose, and w ith an intention? not to be understood ; 

 for he says it is a thing which ought not to come to the 

 knowledge of the vulgar J. What he advances, therefore, 

 being verv true and very easy, as he assures us, and as we 

 have already evinced, we cannot without injustice deny that 

 he possessed this knowledge. ' 



• An Italian palm is between 8 and 9 English inches, and a brasse, or 

 I rice, about 23 English inches. See Dr. Hutton's Dictionary, articles 

 Measure and Palm. 



f Mi lie pass uum =: ml lie pas sa mile = iooo geometrical paces each 

 5 feet = about 534.0 Engfiih feet. — Translator. 



X See S 19. of my second lette .n-lrandaior. 



3 18. «Thu3 



