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 insignificant appendage to the dark ones, thrown in as it were by nature for the purposes of vision. (See Frontispiece, where the space ABCD represents the non-luminous, and CDE the luminous radiation).

The diagram drawn by Professor Müller to represent the distribution of heat in the solar spectrum is not by any means so striking as that just described, and the reason, doubtless, is that prior to reaching the earth the solar rays have to traverse our atmosphere. The aqueous vapour there diffused acts very energetically upon the ultra-red rays, and by it the summit of the peak representing the sun's invisible radiation is cut off. A similar lowering of the mountain of invisible heat is observed when the rays from the electric light are permitted to pass through a film of water, which acts upon them as the atmospheric vapour acts upon the rays of the sun.

7. Combustion by Invisible Rays.

The sun's invisible rays transcend the visible ones in heating power, so that if the alleged performances of Archimedes during the siege of Syracuse had any foundation in fact, the dark solar rays would have