Star-land : being talks with young people about the wonders of the heavens (1891)
Ball, Robert S. (Robert Stawell), Sir, 1840-1913 · London ; Paris : Cassell & Co. · leaf 19
holding a piece of paper at the properdistance a bright spot will be obtained (Fig. 1). At thatspot there is intense heat, by which a match can be lighted,gunpowder exploded, or the paper itself kindled. For thebroad lens collects together the rays from the sun thatfall upon it, and concentrates… … Fig. 2.—The Noonday Gun. they would not be strong enough to burn. But the facesof the lens are specially curved, so that they have theeffect of bendingnin all the rays of light and heat, in sucha way that they are all directed into one point, which we The Sun. 5 call the focus. When a great number…
Astronomy
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