Evening star., 1928-10-28
Reading sky brightness
from roof.

eliminate all sources of error and to as
sure that they are wholly authentic be-
fore their conclusions are made avail
able to the public in general and those
ramifications of art and industry most
concerned.
THE amount of illumination re-
= corded at a given point in room
at any time depends directly for its
intensity on the brightness of the sky
visible from that position through the
intervening window. Hence the bright
ness of the sky is the factor which
governs the natural illumination in al
the buildings that speckle the face OF
our terrestrial sphere. The brightness
of the sky varies widely, as shown by
practical tests made by the United
States Weather Bureau. In some Sec
tions, this brightness is 10 to IL times
greater than at others at the same
moment of given day. In general
the brightest points are close to the
sun, while the minimum brightness Oc-
ours at positions 80 degrees to the
sun. From that point of minimum light
to the horizon, brightness again in
creases very appreciably.
In order to ascertain the true bright
ness of the sky which governs the
natural room illumination at given
station in the test building, bright
ness meter has been mounted on the
roof of the experimental building. 1t
consists of a photo electric cell set up
in rectangular swinging box which is
provided with large metal hood re-
sembling the bellows of the ordinary
camera. This meter is pointed ac-
curately at that section of the sky
whence comes the light that filters into
the test building during an experiment.
1t measures the scientific intensity of
sky brightness at that particular point,
the results being registered on con-
tinuous chart recorder within the glass
house to which it is linked by insulated
wires.
The brightness of the sky for various
ATTITUDES and seasons has been studied
and was published seven years ago by
the Weather Bureau. This comprehen-
sive report shows that, in certain lati-
tudes and attitudes. the average bright
ness of the sky has definite values dur
ing the different seasons.
The Weather Bureau has also made
reliable tests of the moisture content
of the air. as this exerts a measurable
influence on the amount of natural il
Elimination which filters to the desk of
the office worker in a certain we'll
located building. You can appreciate
the Importance of moisture in the at
mosphere from the fact that it varies
as much as IS per cent from day
to day.
At the National Physical Laboratory,
at Teddington, England, British ex-
perimenters have made somewhat sim
liar studies of light values for normal
illumination. However, the building in
which they performed their expert
ments consisted of nothing more than
a skeletal structure without glazed win
dows. The United States Public Health
Service is the first research agency In
history that has experimental with a
building composed largely of glass, in
direct stimulation of the commercial
windows in ordinary use.
1t is nothing more than speculation
to cogitate on the eye-strain and other
sources of defective vision induced in
these United States-and throughout
the world-because builders have not
based their use of window glass and
their design and location of windows on
scientific fact instead of the cut-and-
try method Scientific facts have not
been available, but 1t would
not have been difficult to initiate re-
search in order to determine them.
The authorities of the Public Health