I
now remove the cell, and the inequality of luminosity is still more
glaring. This shows, then, that the rays of maximum luminosity must
travel toward the red as the thickness of the turbid medium is increased.
The observations at 8,000 feet, here recorded, were taken on September
15, at noon, and of course in latitude 46 deg. the sun could not be overhead,
but had to traverse what would be almost exactly equivalent to the
atmosphere at sea level. It is much nearer the calculated intensity for
no atmosphere intervening than it is for one atmosphere. The explanation
of this is easy. The air is denser at sea level than at 8,000 feet up,
and the lower stratum is more likely to hold small water particles or
dust in suspension than is the higher.
[Illustration: FIG. 3.--PROPORTIONS OF TRANSMITTED COLORS.]
For, however small the particles may be, they will have a greater
tendency to sink in a rare air than in a denser one, and less water vapor
can be held per cubic foot. Looking, then, from my laboratory at South
Kensington, we have to look through a proportionately larger quantity of
suspended particles than we have at a high altitude when the air
thicknesses are the same. And consequently the absorption is
proportionately greater at sea level that at 8,000 feet high. This leads
us to the fact that the real intensity of illumination of the different
rays outside the atmosphere is greater than it is calculated from
observations near sea level.
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