This is due to the
absorption, by the water, of a certain portion of the electric beam.
But a portion passes through unabsorbed, and does not at all
contribute to the heating of the water. Now, ice is also in great part
transparent to these latter rays, and therefore is but little melted
by them. Hence, by employing the portion of the beam transmitted by
water, we are able to keep our lens intact, and to produce by means of
it a sharply defined focus. Placed at that focus, white paper is not
ignited, because it fails to absorb the rays emergent from the
ice-lens. At the same place, however, black paper instantly burns,
because it absorbs the transmitted light.
And here it may be useful to refer to an estimate by Newton, based
upon doubtful data, but repeated by various astronomers of eminence
since his time. The comet of 1680, when nearest to the sun, was only a
sixth of the sun's diameter from his surface. Newton estimated its
temperature, in this position, to be more than two thousand times that
of molted iron. Now it is clear from the foregoing experiments that
the temperature of the comet could not be inferred from its nearness
to the sun. If its power of absorption were sufficiently low, the
comet might carry into the sun's neighbourhood the chill of stellar
space.
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