Arabian scientists was Alhazen. His work, published about the
year 1100 A.D., had great celebrity throughout the mediaeval
period. The original investigations of Alhazen had to do largely
with optics. He made particular studies of the eye itself, and
the names given by him to various parts of the eye, as the
vitreous humor, the cornea, and the retina, are still retained by
anatomists. It is known that Ptolemy had studied the refraction
of light, and that he, in common with his immediate predecessors,
was aware that atmospheric refraction affects the apparent
position of stars near the horizon. Alhazen carried forward these
studies, and was led through them to make the first recorded
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