s 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 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