Every one admits the presence of mist on these occasions, and this mist
must be merely a collection of intangible and very minute particles of
suspended water. In a distant landscape we have simply the same or a
smaller quantity of street mist occupying, instead of perhaps 1,000
yards, ten times that distance. Now I would ask, What effect would such a
mist have upon the light of the sun which shone through it?
It is not in the bounds of present possibility to get outside our
atmosphere and measure by the plan I have described to you the different
illuminating values of the different rays, but this we can do: First, we
can measure these values at different altitudes of the sun, and this
means measuring the effect on each ray after passing through different
thicknesses of the atmosphere, either at different times of day or at
different times of the year, about the same hour. Second, by taking the
instrument up to some such elevation as that to which Langley took his
bolometer at Mount Whitney, and so to leave the densest part of the
atmosphere below us.
[Illustration: FIG. 2.--RELATIVE LUMINOSITIES.]
Now, I have adopted both these plans. For more than a year I have taken
measurements of sunlight in my laboratory at South Kensington, and I have
also taken the instrument up to 8,000 feet high in the Alps, and made
observations _there_, and with a result which is satisfactory in that
both sets of observations show that the law which holds with artificially
turbid media is under ordinary circumstances obeyed by sunlight in
passing through our air: which is, you will remember, that more of the
red is transmitted than of the violet, the amount of each depending on
the wave length.
Pages:
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131