nd imperceptibly
toning down into the gray tint of the clouds. The slightest details
were clearly discernible--net, robes, and instruments. Every one of
our gestures was instantaneously reproduced by the aerial spectres.
The anthelion remained upon the clouds sufficiently distinct, and for
a sufficiently long time, to permit of my taking a sketch in my
journal and studying the physical condition of the clouds upon which
it was produced. I was able to determine directly the circumstances of
its production. Indeed, as this brilliant phenomenon occurred in the
midst of the very clouds which I was traversing, it was easy for me to
ascertain that these clouds were not formed of frozen particles. The
thermometer marked 2 deg. above zero. The hygrometer marked a maximum of
humidity experienced, namely, seventy-seven at three thousand seven
hundred and seventy feet, and the balloon was then at four thousand
six hundred feet, where the humidity was only seventy-three. It is
therefore certain that this is a phenomenon of the diffraction of
light simply produced by the vesicles of the mist.

The name of diffraction is given to all the modifications which the
luminous rays undergo when they come in contact with the surface of
bodies. Light, under these circumstances, is subject to a sort of
deviation, at the same time becoming decomposed, whence result those
curious appearances in the shadows of objects which were observed for
the first time by Grimaldi and Newton.

The most interesting phenomena of diffraction are those presented by
_gratings_, as are technically denominated the systems of linear and
very narrow openings situated parallel to one another and at very
small intervals. A system of this kind may be realized by tracing with
a diamond, for instance, on a pane of glass equidistant lines very
close together. As the light would be able to pass in the interstices
between the strokes, whereas it would be stopped in the points
corresponding to those where the glass was not smooth, there is, in
reality, an effect produced as if there were a series of openings very
near to each other. A hundred strokes, about 1/25th of an inch in
length, may thus be drawn without difficulty. The light is then
decomposed in spectra, each overlapping the other. It is a phenomenon
of this kind which is seen when we look into the light with the eye
half closed; the eyelashes in this case, acting as a net-work or
grating. These net-works may also b