Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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But The Illusion Soon Ceased, And We Found That The
Luminous Points Were The Images Of Several Stars Magnified By The
Vapours.
These images remained motionless at intervals, they then
seemed to rise perpendicularly, descended sideways, and returned to
the point whence they had departed.
This motion lasted one or two
seconds. Though we had no exact means of measuring the extent of
the lateral shifting, we did not the less distinctly observe the
path of the luminous point. It did not appear double from an effect
of mirage, and left no trace of light behind. Bringing, with the
telescope of a small sextant by Troughton, the stars into contact
with the lofty summit of a mountain in Lancerota, I observed that
the oscillation was constantly directed towards the same point,
that is to say, towards that part of the horizon where the disk of
the sun was to appear; and that, making allowance for the motion of
the star in its declination, the image returned always to the same
place. These appearances of lateral refraction ceased long before
daylight rendered the stars quite invisible. I have faithfully
related what we saw during the twilight, without undertaking to
explain this extraordinary phenomenon, of which I published an
account in Baron Zach's Astronomical Journal, twelve years ago. The
motion of the vesicular vapours, caused by the rising of the sun;
the mingling of several layers of air, the temperature and density
of which were very different, no doubt contributed to produce an
apparent movement of the stars in the horizontal direction.
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