They usually light it with mirrors made by hollowing the surface of an isosceles right-angled triangle, which conducts all the rays of light into one point. Now when it is placed opposite to the sun, so that all the rays coming from all quarters are collected together into that point, the ray thus formed passes through the thin air, and at once lights the dryest and lightest of the objects against which it strikes, for that ray has the strength and force of fire itself.
