THE DEVELOPMENT OF METEOROLOGY. 
33 
B. C., at Alexandria, down to the time of Copernicus, who 
died in 1543, and of Galileo, who died in 1642. 
To the students of optics we owe the explanation of the twi- 
light, first correctly given by the Arab, Alhazen, who lived 
in Spain in the 11th century, but who may have drawn much 
of his knowledge from earlier Alexandrian Greek manu- 
scripts that are now unknown. But even he knew T nothing 
of the ultimate cause of the refractive power of the atmos- 
phere ; he attributed it to the transparency of the air rather 
than to its density ; whereas Kleomedes, A. D. 50, seemed to 
understand that it was the density of the medium that prin- 
cipally determines the amount of refraction. 
The rainbow and its supplementary bows and halos in 
general w 7 ere observed more or less accurately in the earliest 
ages and are mentioned by Aristotle, w 7 ho knew 7 that they 
depended in some w T ay upon the position of the sun. The 
first steps in the proper explanation of the rainbow were 
taken by Vitello, w-ho began by observing carefully the rain- 
bows formed in the spray of the waterfall at Viterbo; liis 
work on optics was written about 1250, but first published 
in 1572. The complete explanation had to wait for the 
development of theories of the nature of light by Newton, 
Huyghens, Young, and Fresnel; in fact, only within the 
lives of the present generation have Airy, Mascart, and Pern- 
ter perfected our knowledge of halos and rainbows. 
Mirage and the twinkling of the stars were also observed 
and fairly well described by the early waiters in Greece, 
Italy, and Arabia. Pern ter quotes authorities to show- that 
the mirage in the desert, the “Serab,” by which the traveler 
is deceived into thinking that he beholds a distant lake of 
water, is referred to in many old Turkish and Arab docu- 
ments and even in the book of Isaiah. The explanation 
given by the Arabs w 7 as to the effect that the deceptive lake 
of water is due to water-vapor or fog floating over the desert ; 
this error continued until Kepler discovered the phenomenon 
of total reflection of light, which had been independently 
discovered by Newion and given in lectures as early as 1673, 
though his “Optics” was not printed until 1704. 
