in a state to precipitate in larger particles. But when
present as small particles of water the hills look very distant, owing to
what we may call the haze between us and them. In recent weeks every one
has been able to see very multiplied effects of such haze. The ends of
long streets, for instance, have been scarcely visible, though the sun
may have been shining, and at night the long vistas of gas lamps have
shown light having an increasing redness as they became more distant.
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. The luminosity of the spectrum observed at the Riffel I
have used as my standard luminosity, and compared all others with it. The
result for four days you see in the diagram.

I have diagrammatically shown the amount of different colors which
penetrated on the same day