OPTICS. 361 



and Art, and on the Nullity of Magic,' says, " transparent bodies may 



be so figured that things at the greatest distance may appear to be the 



nearest, and the contrary ; so that, from an incredible distance, we 



may read the smallest letters, and number things, however minute : 



thus it is thought that Julius Caesar, on the coast of Gaul, discerned, JuUns csar. 



by or through very large glasses, the disposition and situation of the 



camps and (coast) cities of Britannia Major." We here render per 



ingentia specula, by or through very large glasses, because the author 



is speaking of perspicua, transparent things. On what evidence he 



grounds his assertion we know not. 



The ancients are well known to have used dioptric as well as catop- 

 tric burning-glasses ; and it is not probable that they would employ 

 the former thus, and yet remain ignorant of their magnifying power. 

 The contrary, indeed, is plainly affirmed by Seneca : Liters quamvis Seneca, 

 minutae et obscurae, per vitream pilam aqua plenam, majores clario- A - c - 64 - 

 resque cernuntur. " Letters, though minute and obscure,, appear 

 larger and clearer through a glass bubble filled with water." 1 Such a 

 phenomenon, often observed, would naturally lead inquisitive men a 

 few steps farther. But they could make no important advance (says 

 Dr. Hook) without the art of grinding glass. This they had ; so at 

 least says Pliny. " Some glass is fashioned by blowing ; some is Pliny, 

 ground upon a wheel, or in a turning lathe; and some is engraved A ' 0- ' 9 

 like silver. Sidon was celebrated for its glass-works, having also in- 

 vented specula. Such was the ancient art of glass." 2 What were 

 here meant by specula? The phrase "siquidem etiam specula exco- 

 gitaverat" points evidently to some notable invention. 



That glass was ground by the ancients is also fairly deducible from 

 the language of Seneca. He tells us that prisms were in use among 

 the virtuosi of Rome, in the days of Nero ; and how could a glass 

 prism be made by blowing? " A rod, or bar of glass (says he) is 

 made with several angles ; and if the rays of the sun pass through it, 

 such colours are made as we see in the rainbow." 3 Seneca also speaks 

 of multiply ing-glasses, the several faces of which must have been cut 

 upon a wheel. 



All this, however, brings us not to any such invention as that of 

 telescopes. Nor are we aware of anything in antiquity that can indu- 

 bitably be so interpreted. We have seen adduced, for this purpose, a 

 passage from Pisidas, a Christian writer, who flourished at Constan- 

 tinople in the seventh century : Ta /zeXXovra WQ 8ia c)io7rrpou av A - c - 67 * 

 /3\7rtc 5 " you see things future as by a dioptrum" They who con- 

 tend for the early invention of telescopes, say that this dioptrum was 

 a prospective glass. But, if it were the same as the dioptrum em- 

 ployed by Hipparchus, and afterwards by Ptolemy, it was no other 

 than a straight ruler of about four feet long, on which were fixed 



1 Seneca, Nat. Quast. lib. i. c. 7. 



2 Plin. Nat. Hist. lib. xxxvi. c. 26. 



3 Seneca, Nat. Qusest. lib. i. c. 7. 



