stances. During two months' observation in the pure air of
Mount Sherman (8,335 feet high) in the summer of 1872, these tell-tale
lines mounted up to 273;[600] and he believes their number might still
be doubled by steady watching. Indeed, both Young and Lockyer have more
than once seen the whole field of the spectroscope momentarily inundated
with bright rays, as if the "reversing layer" had been suddenly thrust
upwards into the chromosphere, and as quickly allowed to drop back
again. The opinion would thus appear to be well-grounded that the two
form one continuous region, of which the lower parts are habitually
occupied by the heaviest vapours, but where orderly arrangement is
continually overturned by violent eruptive disturbances.

The study of the _forms_ of prominences practically began with Huggins's
observation of one through an "open slit" February 13, 1869.[601] At
first it had been thought possible to examine them only in
sections--that is, by admitting mere narrow strips or "lines" of their
various kinds of light; while the actual shape of the objects emitting
those lines had been arrived at by such imperfect devices as that of
giving to the slit of the spectroscope a vibratory moment rapid enough
to enable the eye to retain the impression of one part while others were
successively presented to it. It was an immense gain to find that their
rays had strength to bear so much of dilution with ordinary light as was
involved in opening the spectroscopic shutter wide enough to exhibit the
tree-like, or horn-like, or flame-shaped bodies rising over the sun's
rim in their undivided proportions. Several diversely-coloured images of
them are formed in the spectroscope; each may be seen under a crimson, a
yellow, a green, and a deep blue aspect. The crimson, however (built up
out of the C-line of hydrogen), is the most intense, and is commonly
used for purposes of observation and illustration.

Friedrich Zoellner was, by a few days, beforehand with Huggins in
describing the open-slit method, but was somewhat less prompt in
applying it. His first survey of a complete prominence, pictured in, and
not simply intersected by, the slit of his spectroscope, was obtained
July 1, 1869.[602] Shortly afterwards the plan was successfully adopted
by the whole band of investigators.

A difference in kind was very soon perceived to separate these objects
into two well-marked classes. Its natural and obvious character was
shown by