the time of Aristophanes, who mentions it in the ' Clouds,' in a dialogue between Socrates and Strepsiades:—
"Socrates. Very good: now I'll set yon another smart question. If some one entered an action against you to recover five talents, tell me, how would you cancel it?
Strepsiades. I have found a very clever way to cancel the suit, as you will agree yourself.
Socrates. What kind of a way?
Strepsiades. Have you ever seen that stone in the druggists' shops, that pretty, transparent one, that they light fire with?
Socrates. The crystal, you mean?
Strepsiades. I do.
Socrates. Well, what then?
Strepsiades. Suppose I take this, and when the clerk enters the suit, I stand thus, a long way off. towards the sun, and melt out the letters.
Socrates. Very clever, by the Graces! "[1]
At a much later period Pliny mentions that glass balls with water put into them, when set opposite to the sun, get so hot as to set clothes on fire; and that he finds surgeons consider the best means of cautery to be a crystal ball placed opposite to the sun's rays.[2] The Chinese commonly use the burning-lens to light fire with, as well as the flint and steel, and we hear of the Siamese using it to produce new sacred fire.[3]
The fact that fire may be produced by reflecting the sun's rays with mirrors was known as early as Pliny's time (A.D. 23-79), as he remarks, "seeing that concave mirrors placed opposite to the sun's rays ignite things more easily than any other fire."[4] There is some reason to suppose that the knowledge of this phenomenon worked backwards into history, attaching itself to two famous names of old times, Archimedes and Numa Pompilius. The story of Archimedes setting the fleet on fire at Syracuse with burning mirrors, probably un- known as it was to historians for centuries after his time, need not be further remarked on here; but the story of Numa reappears on the other side of the world, under circumstances which make its discussion a matter of importance to ethnography.
