Watertown republican. [volume] (Watertown, Wis.) 1860-1906, 1873-08-20
Sunbeams and Shadows.

1t is easy to dar1en room, even in one
of the. brightest of days, by closing the
shutters, and at the same time, if need be,
by hanging shawls or blankets over the
windows. Ifa small hole is left in the COW
ering of one Of the windows, to let the
sunlight through, the path of the sun
beam in the room may be distinctly seen.
If the air is sprinkled with dust, as it may
be by shaking dusty cloth, the beam is
peculiarly bright and beautiful, and no
artist could possibly draw line SO alb
solutely straight as it is seen to be. We
often see such beams coming through our
half-closed window shutters, and streak
ing the dusty air Of our rooms with bars
Of Hght-beautiful illustrations of the fact
that light always moves in straight lines.
When the sun is sinking behind clouds
in the western sky, it often presents an
appearance very like that seen in our
haltdarkened rooms, and which is some
times described by saying that the "sun
draws water" Is it possible that any
one ever believed the streaks he saw were
really streams of water being pulled up
by the sun? We know now that they are
beams Of light from the sun, which is
shining through openings in the Clouds
They are made in just the same
way as are the beams 81 of light stseen
in OH rooms when the sun shines through
openings in the window shutters
1t is also upon the same principle that
shadows are made. If you would ex-
anime the case yourself, you can proceed
as follows: Put book, vertically, at
convenient distance in front of lamp
flame. Now some of the rays of the
light fall upon the book and cannot go
further, but others just Craze the edges of
the book and pass in straight lines onward,
so that behind the book, and reaching to
the opposite wall, is a space from which
the light is shut out. This space which
is deprived of light is the shadow of the
book. s4sa @ .i hA@
We are apt to call the black surface on
the wall the shadow, but the shadowreally
reaches all the way from the book to the
from which the light is shut out, and the
black spot on the wall is only one end
of it. s-ts.4.z j
If we examine the shadows made by
the various objects around the evening
lamp, as they appear upon the wall, we
can hardly fail to see that each one is
made up of two parts. Every shadow
has dark middle part, surrounded by
lighter portion. The lighter portion is
often very narrow, a mere fringe indeed,
but it is seldom entirely absent.
A very easy experiment tells us how
these two parts of shadow are produced.
Place two lamp-fames upon fable near
each other, and hold a lead pencil, or
narrow strip of wood, between the flames
and sheet of paper, at some distance
from them. Two distinct shadows will be
seen, one cast by each light. Now move
the pencil nearer to the paper The shad-
ows will approach each other, until, at
last they overlap and form one, in which
the umbra and the penumbra, as the two
parts of shadow are called, may be seen
with surprising clearness. The umbra
gets no light from either flame, but it is
easy to see that every part of the penum-
bra is getting light from one or the other.
On this account the umbra is darker than
the penumbra. m
Now in the shadow cast, when single
flame is used, the outer parts are getting
light from one edge or another of the flame,
while the middle part is getting no light
at all from any portion of the flame For
this reason the middle part, or umbra, is
darker than the penumbra. Elements Of
Natural Philosophy. -