Omaha daily bee., 1914-01-03


cause or source of daylight but 1t does
not say that daylight and sunlight are
identical In fact, they differ almost as
much as do water and wine.
But the degree of difference varies.
DayHghtais mixture Of two kinds of
light and it's quality is continually
changing, as everybody who has ever
had anything to do with photography
knows The proportions of the two kinds
of light that make daylight are not the
same from hour to hour, and hardly
from minute to minute.
In clear weather, under an open sky,
with the sun high in the heavens, day.
light, says Prof. Nichols OF Cornell, Is
almost entirely sunlight. A white surface
sexposed to an unobscured sky receives
directly from the sun So per cent OF the
light that illuminates 1t and only 10 per
cant from other parts of the sky dome
This other part of the illumination is
tcalled skylight and II consists of light
nearly all of which came originally from
the sun, but which has been changed In
.ouality by reflection from the earth, from
the clouds and from dust and vapor IN
the air. Many of the rays that character
lied the original sunlight have been ao-
sorbed by the reflecting substances so
that what remains is no longer the same
thing as before.
= ON a completely overcast day there is
mo sunlight, properly so called, but only
.kky-Iight. Whether the sky Is overcast
or not the intensity OF daylight varies
with the hour OF the day and with the
season. This is Que to differences in the
elevation OF the sun. These variations in
the intensity OF daylight are surprisingly
great. The intensity is, on the average,
zen times as great in midsummeI as in
.midwinter, but this average comes far
sfrom expressing the utmost difference
1tbat can exist, for investigation has
.sbown that between the clearest summer
sdav and the darkest day of winter the
sratio Of the intensity OF daylight may be
as great as 800 to /.
Besides, the quality Of daylight is con-
Ttinually changing on account OF the
va riatlons IN the relative amounts Of the
different rays of the spectrum that are
Mingled in IT, The spectrum OF light is
A gamut Of vibration, and the result Of
the selective action exercised by the sub
stances and Vapors, from which the light
has been reflected and through which II
has passed is to product variations OF
ecolor and of intensity OF color, as well as
OF the quantity of the invisible radiations
present, and these variations are not the
less real and important because the eye
is not always aware Of them.
. In fact, one OF the most astonishing
things about the human eye Is the vast
extent of its sensibility to light combined
with its ability to bear enormous changes
Of intensity without losing it's power OF
seeing. On a cIoudless midsummer day
: the light is 5000 OF 5000 times as intense
I as the illumination that the eye needs tc
read ordinary print, and yet the eye Il
notgblasted by this excess of light. Ye
Il approaches the danger limit
The eye. says Prof. Nichols, must serve
,us not only at midnight, but also II
ttFa.morning, in the evening, indoors and
during the night. II is never normally ex
posed 10 a more intense light than tha
OF full day and, consequently. we fiu'
that the extent OF its sensibility is sucl
that fuii Daylight represents the eXtrem'