BOOK XXXIII. xLiv. 127-XLV. 129 



as good, while if it turns red it is of the next quality, 

 and if black it has no value at all. But fraud has 

 found its way even into this test ; if the shovels are 

 kept in men's urine the silver shaving is stained by 

 it during the process of being burnt, and counter- 

 feits ^vhiteness. There is also one way of testing 

 polished silver in a man's breath — if it at once forms 

 surface moisture and dissipates the vapour. 



XLV. It has been believed that only the best Rejieaing 

 silver is capable of beinof beaten out into plates ?"<^«'»f'» <^'"-' 

 and producmg an image. 1 nis was lormeriy a siiver. 

 sound test, but nowadays this too is spoiled by fraud. *""^*- 



Still, the property of reflecting images is marvel- 

 lous ; it is generally agreed that it takes place owing 

 to the repercussion of the air which is thrown back 

 into the eyes. In a similar way, owing to the same 

 force, in employing a mirror if the thickness of the 

 metal has been poHshed and beaten out into a sHghtly 

 concave shape the size of the objects reflected is 

 enormously magnified : such a difference does it 

 make whether the surface welcomes ^ the air in 

 question or flings it back.^ Moreover bowls can be 

 made of such a shape, with a number of looking- 

 glasses so to speak beaten outward inside them, 

 that if only a single person is looking into them a 

 crowd of images is formed of the same number as the 

 facets in question. Ingenuity even devises vessels 

 that do conjuring tricks, for instance those deposited 

 as votive offerings in the temple at Smyrna : this is 

 brought about by the shape of the material, and it 

 makes a very great difference whether the vessels 

 are concave and shaped hke a bowl or convex like a 

 Thracian shield, whether their centre is recessed or 

 projecting, whether the oval ^ is horizontal or obhque, 



97 



