eflectors. They serve to give us a diffused light which enables us to
see clearly all visible objects. We have all noticed the effect of a
single electric arc light, situated at a distance from any other source
of light, and how it casts extremely dark shadows and very high lights;
so much so that it is difficult to see an object perfectly in this
light, because the part of an object that is under the direct rays of
the lamp is so highly illuminated that the shadow, by comparison, has
the effect of simply a dark blot without form or shape. Many of you
have noticed in a country village, where the streets are lighted with
electric arc lamps, what a difference there is in the illuminating
effect between a clear and a foggy night. When there is a fog, or when
the clouds hang low down, we get a reflection from these which tends to
diffuse and soften the powerful light rays that are sent out by these
lamps. This effect is especially noticeable when the night is only
moderately foggy. Each globule of moisture floating in the air becomes a
reflector of light, and by myriads of reflections and counter
reflections the light (which on a clear night is concentrated) is
diffused over a large area, producing an illumination which for
practical purposes is far superior to that produced on a clear night.
When the latter condition prevails the rays of light are so intense on
objects immediately surrounding the lamps that one is blinded; so that
the places which are in shadow seem darker than they would be if there
were no light at all. The only way to prevent this effect is to have the
lights so close together that there will be cross lights, which tend to
break up the intensity of the shadows. This principle of light diffusion
is taken advantage of to produce an even illumination in stores that are
lighted only on one or two sides. This is effected by a series of prisms
or reflecting surfaces that are cast upon the panes of glass.

If now there were no atmosphere--or, to state it differently--if there
were no floating substances in the atmosphere, the sun would produce an
effect upon the earth similar to that of a single electric light. The
lights would be extremely high, and the shadows extremely dense. To one
looking off into space, the sky, instead of having the blue appearance
that we see, would have the effect of looking into a deep, dark abyss
without illumination.

Tyndall has shown us by a beautiful experiment that if there be