aid to be white light and its spectrum indicates the
presence of visible radiant energy of all wave-lengths in approximately
equal proportions. North skylight contains an excess of violet, blue,
and blue-green rays and as a consequence is a bluish white. Noon
sunlight on a clear day is fairly constant in spectral character, but
north skylight varies somewhat depending upon the absence or presence of
clouds and upon the character of the clouds. If large areas of sunlit
clouds are present, the light is largely reflected sunlight. If the sky
is overcast, the north skylight is a result of a mixture of sunlight and
blue skylight filtered through the clouds and is slightly bluish. If the
sky is clear, the light varies from light blue to deep blue.

[Illustration: FIREWORKS AND ILLUMINATED BATTLE-FLEET AT HUDSON-FULTON
CELEBRATION]

[Illustration: FIREWORKS EXHIBITION ON MAY DAY AT PANAMA-PACIFIC
EXPOSITION]

The daylight which enters buildings is often considerably altered in
color by reflection from other buildings and from vegetation, and after
it enters a room it is sometimes modified by reflection from colored
surroundings. It may be commonly noted that the light reflected from
green grass through a window to the upper part of a room is very much
tinted with green and the light reflected from a yellow brick building
is tinted yellow. Besides these alterations, sunlight varies in color
from the yellow or red of dawn through white at noon to orange or red at
sunset. Throughout the day the amount of light from the sky does not
change nearly as much as the amount of sunlight, so there is a
continual variation in the proportion of direct sunlight and skylight
reaching the earth. This is further varied by the changing position of
the sun. For example, at a north window in which the direct sunlight may
not enter throughout the day, the amount of sunlight which enters by
reflection from adjacent buildings and other objects may vary greatly.
Thus it is seen that daylight not only varies in quantity but also in
quality, and an artificial daylight, which is based upon an extensive
analysis, has the advantage of being constant in quantity and quality as
well as correct in quality. Modern artificial-daylight units which have
been scientifically developed not only make mankind independent of
daylight in the discrimination of colors but they are superior to
daylight.

Although there are many expert colorists who require an accurate