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. Prof. Langley, in this theater, in a
remarkable and interesting lecture, in which he described his journey up
Mount Whitney to about 12,000 feet, told us that the sun was really blue
outside our atmosphere, and at first blush the amount of extra blue which
he deduced to be present in it would, he thought, make it so. But though
he surmised the result from experiments made with rotating disks of
colored paper, he did not, I think, try the method of using pure colors,
and consequently, I believe, slightly exaggerated the blueness which
would result.

I have taken Prof. Langley's calculations of the increase of intensity
for the different rays, which I may say do not quite agree with mine, and
I have prepared a mask which I can place in the spectrum, giving the
different proportions of each ray as calculated by him, and this when
placed in front of the spectrum will show you that the real color of
sunlight outside the atmosphere, as calculated by Langley, can scarcely
be called bluish. Alongside I place a patch of light whi