ipsed sun. In its original
form, it is true, it came to an end when Professor Harkness, in
1869,[553] pointed out that the shadow of the moon falls equally over
the air and on the earth, and that if the sun had no luminous
appendages, a circular space of almost absolute darkness would
consequently surround the apparent places of the superposed sun and
moon. Mr. Proctor,[554] with his usual ability, impressed this
mathematically certain truth upon public attention; and Sir John
Herschel calculated that the diameter of the "negative halo" thus
produced would be, in general, no less than 23 deg.

But about the same time a noteworthy circumstance relating to the state
of things in the solar vicinity was brought into view. On February 11,
1869, Messrs. Frankland and Lockyer communicated to the Royal Society a
series of experiments on gaseous spectra under varying conditions of
heat and density, leading them to the conclusion that the higher solar
prominences exist in a medium of excessive tenuity, and that even at the
base of the chromosphere the pressure is far below that at the earth's
surface.[555] This inference was fully borne out by the researches of
Wuellner; and Janssen expressed the opinion that the chromospheric gases
are rarefied almost to the degree of an air-pump vacuum.[556] Hence was
derived a general and fully justified conviction that there could be
outside, and incumbent upon the chromosphere, no such vast atmosphere as
the corona appeared to represent. Upon the strength of which conviction
the "glare" theory entered, chiefly under the auspices of Sir Norman
Lockyer, upon the second stage of its existence.

The genuineness of the "inner corona" to the height of 5' or 6' from the
limb was admitted; but it was supposed that by the detailed reflection
of its light in our air the far more extensive "outer corona" was
optically created, the irregularities of the moon's edge being called in
to account for the rays and rifts by which its structure was varied.
This view received some countenance from Admiral Maclear's observation,
during the eclipse of 1870, of bright lines "everywhere"--even at the
centre of the lunar disc. Here, indeed, was an undoubted case of
atmospheric diffusion; but here, also, was a safe index to the extent of
its occurrence. Light scatters equally in all directions; so that when
the moon's face at the time of an eclipse shows (as is the common case)
a blank in the spectroscope, it is quit