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88 THROUGH NORWAY WITII A KNAPSACK.
that, under these circumstances, an apparent undulating
motion would be given to objects on the horizon; they
would appear to rise or fall, according as he viewed
them through a denser or a rarer atmosphere; and thus
the waving of the coils of the serpent’s body is
accounted for. This may be illustrated by holding a hot
poker between the eye and a distant object which is
seen just over the poker.
But how about the undermining of the head-rock
forming the serpent’s uplifted jaw? This is as easily
accounted for, though the principles upon which it
depends are not so popularly understood. One of
the most beautiful illustrations of the mathematical
consistency—so to speak—of Nature’s laws, is a curious
consequence of the law, that in a given body the
sine of the angle of ordinary refraction bears a fixed
ratio to that of the angle of incidence. I will not
enter into the mathematics of this, but merely state the
fact; which is, that light cannot pass at all from a dense
into a rare medium at angles exceeding a certain degree
of obliquity, as the law of refraction could not, under
such circumstances, be fulfilled; thus all the light is
turned back, for if it cannot go through in its own way
it won’t go through at all. Ilence under certain
circumstances the thinnest film of air is absolutely opaque:
more opaque than a dense metal, for gold leaf allows
some light to pass through it. Such a film of air
admitting the passage of no light whatever, but reflecting
all that falls upon it, shines like the brightest polished
silver.
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