292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
on optics was written about 1250, but first published by Risner in 
1572. The complete explanation had to wait for the development of 
theories of the nature of ight by Newton, Huyghens, Young, and 
Fresnel ; in fact, only within the lives of the present generation have 
Airy, Mascart, Pernter, and Tanakadate perfected our knowledge of 
halos and rainbows. 
Mirage and the twinkling of the stars were also observed and fairly 
well described by the early writers in Greece, Italy, and Arabia. 
Pernter quotes authorities to show that the mirage in the desert, the 
“ Serab,” by which the traveler is deceived into thinking that he be- 
holds a distant lake of water, is referred to in many old Turkish and 
Arab documents and even in the book of Isaiah. The explanation 
given by the Arabs was to the effect that the deceptive lake of water 
is due to water vapor or fog floating over the desert; this error con- 
tinued until Kepler discovered the phenomenon of total reflection of 
light, which had been independently discovered by Newton and given 
in lectures as early as 1673, though his “ Optics” was not printed 
until 1704. 
The chemical composition of the atmosphere was scarcely suspected 
or suggested by any of the ancient writers, and we must come down 
to the days of Priestley, Scheele, and Lavoisier to find anything 
known on this subject. 
The idea that air has physical properties, such as mass or weight, 
and that it could offer material resistance to bodies passing through 
it, was often expressed, but the properties were not satisfactorily ob- 
served and measured until the days of Galileo in Italy and Stevin in 
Holland. Galileo, having a pump for compression, was able to show 
that air is a compressible gas; but having no means of pumping the 
air out of a receiver, he was unable to entertain the idea of a vacuum, 
and in fact explained the rise of water in a pump as due to the horror 
of a vacuum, until his pupil, Torricelli, presented the idea of the 
elastic pressure of the atmosphere. 
Other mechanical properties of the gases of the atmosphere, such 
as inertia, centrifugal force, expansion with heat, density, elastic re- 
sistance, and viscosity, were entirely unknown to the ancients, and 
were first clearly set forth by Galileo, Torricelli, Stevin, Descartes, 
Huyghens, Hook, Boyle, and Sir Isaac Newton. 
SOCIETIES FOR RESEARCH IN METEOROLOGY. 
The association of men into academies or some equivalent organiza- 
tions dates back to the remotest history. The wise men or learned 
priests and philosophers of Persia, Assyria, and Egypt were organ- 
ized in companies connected with temples of worship and as official 
astrologers in connection with the astronomical observatories. The 
