burning glass

I took the precaution, however, to put the burning-glass into my pocket lest we should want fire.

They lack the burning glass of a purpose, to focalize upon one  spot the separate rays of their ability.

He was like a great burning glass, concentrating the rays of the sun upon a single spot; he burned a hole wherever he went.

To show the action of a burning glass send a beam of light into a darkened room, and place in its path a double convex lens.

That burning glass is what did it, said one more reflective man.