A History of Science, vol 2 | Page 9

scientific estimate of the phenomena of twilight and of the height of the
atmosphere. The persistence of a glow in the atmosphere after the sun
has disappeared beneath the horizon is so familiar a phenomenon that
the ancient philosophers seem not to have thought of it as requiring an
explanation. Yet a moment's consideration makes it clear that, if light

travels in straight lines and the rays of the sun were in no wise
deflected, the complete darkness of night should instantly succeed to
day when the sun passes below the horizon. That this sudden change
does not occur, Alhazen explained as due to the reflection of light by
the earth's atmosphere.
Alhazen appears to have conceived the atmosphere as a sharply defined
layer, and, assuming that twilight continues only so long as rays of the
sun reflected from the outer surface of this layer can reach the spectator
at any given point, he hit upon a means of measurement that seemed to
solve the hitherto inscrutable problem as to the atmospheric depth. Like
the measurements of Aristarchus and Eratosthenes, this calculation of
Alhazen is simple enough in theory. Its defect consists largely in the
difficulty of fixing its terms with precision, combined with the further
fact that the rays of the sun, in taking the slanting course through the
earth's atmosphere, are really deflected from a straight line in virtue of
the constantly increasing density of the air near the earth's surface.
Alhazen must have been aware of this latter fact, since it was known to
the later Alexandrian astronomers, but he takes no account of it in the
present measurement. The diagram will make the method of Alhazen
clear.
His important premises are two: first, the well-recognized fact that,
when light is reflected from any surface, the angle of incidence is equal
to the angle of reflection; and, second, the much more doubtful
observation that twilight continues until such time as the sun, according
to a simple calculation, is nineteen degrees below the horizon.
Referring to the diagram, let the inner circle represent the earth's
surface, the outer circle the limits of the atmosphere, C being the earth's
centre, and RR radii of the earth. Then the observer at the point A will
continue to receive the reflected rays of the sun until that body reaches
the point S, which is, according to the hypothesis, nineteen degrees
below the horizon line of the observer at A. This horizon line, being
represented by AH, and the sun's ray by SM, the angle HMS is an angle
of nineteen degrees. The complementary angle SMA is, obviously, an
angle of (180-19) one hundred and sixty-one degrees. But since M is
the reflecting surface and the angle of incidence equals the angle of
reflection, the angle AMC is an angle of one-half of one hundred and
sixty-one degrees, or eighty degrees and thirty minutes. Now this angle

AMC, being known, the right-angled triangle MAC is easily resolved,
since the side AC of that triangle, being the radius of the earth, is a
known dimension. Resolution of this triangle gives us the length of the
hypotenuse MC, and the difference between this and the radius (AC),
or CD, is obviously the height of the atmosphere (h), which was the
measurement desired. According to the calculation of Alhazen, this h,
or the height of the atmosphere, represents from twenty to thirty miles.
The modern computation extends this to about fifty miles. But,
considering the various ambiguities that necessarily attended the
experiment, the result was a remarkably close approximation to the
truth.
Turning from physics to chemistry, we find as perhaps the greatest
Arabian name that of Geber, who taught in the College of Seville in the
first half of the eighth century. The most important researches of this
really remarkable experimenter had to do with the acids. The ancient
world had had no knowledge of any acid more powerful than acetic.
Geber, however, vastly increased the possibilities of chemical
experiment by the discovery of sulphuric, nitric, and nitromuriatic acids.
He made use also of the processes of sublimation and filtration, and his
works describe the water bath and the chemical oven. Among the
important chemicals which he first differentiated is oxide of mercury,
and his studies of sulphur in its various compounds have peculiar
interest. In particular is this true of his observation that, tinder certain
conditions of oxidation, the weight of a metal was lessened.
From the record of these studies in the fields of astronomy, physics,
and chemistry, we turn to a somewhat extended survey of the Arabian
advances in the field of medicine.
ARABIAN MEDICINE
The influence of Arabian physicians rested chiefly upon their use of
drugs rather than upon anatomical knowledge. Like the mediaeval
Christians, they looked with horror on dissection of