Year: 1876

THE straight lines in which light vibra-
tionS move are called rays. So any body
placed in frCnt of a luminous object at
once cuts off the light. At the same time
the side of the object turned toward the
light is illuminated, while the opposite
side remains dark. Commonly the image
of body falling upon plane surface, as
the ground, is called its shadow. The
| shadow, however, as really exists at any
point behind the body as at the plane.
A shadow is only portion of space in
which there are no light vibrations, and
consequently its intensity varies with the
intensity of the source of light. The
feeble candle makes very faint shadow,
whose outlines are not we'll defined, while
an intense light, like the electric lamp or
the sun, makes very black shadow, be-
cause the contrast between the illuminated
objects and those in darkness is very
great. In open sunlight, indeed, the
diffused daylight everywhere present
modifies the intensity of the shadow, so
that natural shadows do not have any very
marked edges.


The shape of the shadows is plainly the
same as the shape of the objects casting
them, while their size depends upon the
distance of the objects from the light
Besides, when the light is smaller than
the body the shallow constantly increases
in size.


The connection of a shadow with
eclipses deserves special notice. The
sun is the luminous body, and is many
times the size of the body casting the
shadow. Lines drawn tangent to the sun
and the Opaque body determine the
shadow, which is now cone. When one
Opaque body moves into the shadow of
another all the light is cut off from it and
it is eclipsed, while the sun is hidden from
1t and is also eclipsed.


The lightthat falls upon the Illuminated
side of an object is reflected from it in all
directions. This enables us to see the OBJ
the shadow, enables us to Judge of its
shape and size. 1t also produces the diff
fused light of our rooms. Without it OBJ
jects could be seen only when in direct
sunlight, and our rooms would be totally
dark except when the suns rays entered
them, while the contrast between light and
shade would be almost insufferable.


A perfectly smooth surface reflects light
only in single direction. This gives an
image of the object which emits the light
and forms a plane mirror. A perfect
mirror would be invisible, and would only
show the light placed as far behind it as
the light is in front of it. But none of
our mirrors are perfect, and all give suf-
ficient ditfused light to be seen readily. A
curved mirror is only an infinite number
of plane mirrors combined so as to reflect
all the light to single point. Curved
mirrors are applied to rejecting a beam
of light TO illuminate the track in front of
locomotive, when the inside of hollow
cone is used, and to collect the rays of
light from distant object, as in the re-
fiecting telescope, when the mirror is a
hollow sphere.


The laws of reflection are the same for
all cases, and apply only to each ray by
itself. They are two in number.


1. The incident ray, the perpendicular
to the surface at the point where the ray
meets it, and the reflected rays are all in
the same plane.


2. The angle between the incident ray
and the perpendicular, called the angle
of incidence, equals the angle between
the perpendicular and the reflected ray,
and is called the angle of reflection
Boston Wdtc,radn.