ances were made, in computing relative intensities, for atmospheric
ravages on sunlight, for the extra impediments to its passage presented
by the smoke-laden air of Pittsburgh, or for the obliquity of its
incidence. Thus, a very large balance of advantage lay on the side of
the metal.

A further element of uncertainty in estimating the intrinsic strength of
the sun's rays has still to be considered. From the time that his disc
first began to be studied with the telescope, it was perceived to be
less brilliant near the edges. Lucas Valerius, of the Lyncean Academy,
seems to have been the first to note this fact, which, strangely enough,
was denied by Galileo in a letter to Prince Cesi of January 25,
1613.[726] Father Scheiner, however, fully admitted it, and devoted some
columns of his bulky tome to the attempt to find its appropriate
explanation.[727] In 1729 Bouguer measured, with much accuracy, the
amount of this darkening; and from his data, Laplace, adopting a
principle of emission now known to be erroneous, concluded that the sun
loses eleven-twelfths of his light through absorption in his own
atmosphere.[728] The real existence of this atmosphere, which is totally
distinct from the beds of ignited vapours producing the Fraunhofer
lines, is not open to doubt, although its nature is still a matter of
conjecture. The separate effects of its action on luminous, thermal, and
chemical rays were carefully studied by Father Secchi, who in 1870[729]
inferred the total absorption to be 88/100 of all radiations taken
together, and added the important observation that the light from the
limb is no longer white, but reddish-brown. Absorptive effects were thus
seen to be unequally distributed; and they could evidently be studied to
advantage only by taking the various rays of the spectrum separately,
and finding out how much each had suffered in transmission.

This was done by H. C. Vogel in 1877.[730] Using a polarising
photometer, he found that only 13 per cent. of the violet rays escape at
the edge of the solar disc, 16 of the blue and green, 25 of the yellow,
and 30 per cent. of the red. Midway between centre and limb, 88.7 of
violet light and 96.7 of red penetrate the absorbing envelope, the
abolition of which would increase the intensity of the sun's visible
spectrum above two and a half times in the most, and once and a half
times in the least refrangible parts. The nucleus of a small spot was
ascertained to be of the