182 SCIENCE IN GRECO-ROMAN ANTIQUITY 



without leaving any space between. Moreover the 

 force of attraction is not at all the same between all 

 these elements. It is little felt between a liquid and 

 a solid, but it is felt much more between a liquid and 

 the air. This is why the air sucked up out of a tube 

 half plunged in water attracts the water strongly and 

 counterbalances its weight. There is equilibrium when 

 the weight of the column of water raised is equal to 

 the force of the attraction of the air. 



The Ancients also admitted that sound is propagated 

 in the air by spherical waves (Vitruvius, de architect., 

 Bk. V), and that it can be sent back by an obstacle 

 and produce an echo. 1 



They admitted as well that light is propagated in 

 a straight line, and that it is reflected on a polished 

 surface at an angle equal to the angle of incidence. 

 This law seems to have been known by Plato, judging 

 by certain passages in the Timaeus (45 B and parti- 

 cularly 46 B) ; it was clearly enunciated by Euclid, 

 who demonstrated its principal consequences (Euclid, 

 VII, Optica). Refraction was also studied, chiefly by 

 Ptolemy. a 



The property possessed by concave mirrors of giving 

 an enlarged image of an object was certainly utilized. 

 The Ancients were also acquainted with magnifying 

 lenses, although they did not know how to combine 

 them for the construction of telescopes or binoculars 

 or even eye-glasses. In the Clouds of Aristophanes 

 (Act II, Scene 1) Strepsiades undertakes to efface by 

 means of a lens the characters engraved on a tablet 

 of wax : " When the registrar has written his summons 

 against me, I shall take the glass and standing thus 

 in the sun, I shall make his writing melt." Seneca, 



1 A. de Rochas, La Science des philosophes et I'art des thau- 

 maturges dans Vantiquite, Dorbon Aine, Paris, pp. 35 and 39. 



2 On the beginnings of mathematical physics, see 17 Loria, 

 Scienze esatte, p. 557. 



