e produced by reflection, and it is
to this circumstance that are due the brilliant colors observed when a
pencil of luminous rays is reflected on a metallic surface regularly
striated.

To the phenomena of gratings must be attributed, too, the colors,
often so brilliant, to be seen in mother-of-pearl. This substance is
of a laminated structure; so much so, that in carving it the different
folds are often cut in such a way as to form a regular net-work upon
the surface. It is, again, to a phenomenon of this sort that are due
the rainbow hues seen in the feathers of certain birds, and sometimes
in spiders' webs. The latter, although very fine, are not simple, for
they are composed of a large number of pieces joined together by a
viscous substance, and thus constitute a kind of net-work.

If the sun is near the horizon, and the shadow of the observer falls
upon the grass, upon a field of corn, or other surface covered with
dew, there is visible an aureola, the light of which is especially
bright about the head, but which diminishes from below the middle of
the body. This light is due to the reflection of light by the moist
stubble and the drops of dew. It is brighter about the head, because
the blades that are near where the shadow of the head falls expose to
it all that part of them which is lighted up, whereas those farther
off expose not only the part which is lighted up, but other parts
which are not, and this diminishes the brightness in proportion as
their distance from the head increases. The phenomenon is seen
whenever there is simultaneously mist and sun. This fact is easily
verified upon a mountain. As soon as the shadow of a mountaineer is
projected upon a mist, his head gives rise to a shadow surrounded by a
luminous aureola.

[Illustration: FOG-BOW SEEN FROM THE MATTERHORN.]

_The Illustrated London News_ of July 8, 1871, illustrates one of
these apparitions, "The Fog-Bow, seen from the Matterhorn," observed
by E. Whymper in this celebrated region of the Alps. The observation
was taken just after the catastrophe of July 14, 1865; and by a
curious coincidence, two immense white aerial crosses projected into
the interior of the external arc. These two crosses were no doubt
formed by the intersection of circles, the remaining parts of which
were invisible. The apparition was of a grand and solemn character,
further increased by the silence of the fathomless abyss into which
the four ill-fated tourists had j