TOTAL ECLIPSE OF THE SUN, APRIL 16, 1893. 
607 
of the sun with which I am acquainted. The Italian observations of the quiet 
solar atmosphere and the Kensington observations of sun-spots may be especially 
mentioned. Not only is there no correspondence in intensity, but the variation in 
the sun-spot spectrum from maximum to minimum is enormous, while the Fraunhofer 
lines remain constant. 
The Brightness of the Arcs at Different Levels. 
The existence of a thin stratum competent to produce the Fraunhofer spectrum 
being thus disproved by the eclipse photographs, we have next to see if there are any 
indications as to the localization of the absorbing vapours which are not represented 
in the base of the chromosphere. 
The most direct evidence which the eclipse photographs give as to the separation 
of the solar atmospheric vapours into layers is that afforded by the increased relative 
brightness of some of the lines in passing to higher levels. 
As we have to deal with the projection of a sphere and not with a section of the 
sun’s atmosphere, the spectrum arcs would brighten in passing outwards from the 
photosphere in consequence of the increased thickness of vapour presented to us, even 
if the radiation per unit volume remained constant. The spectroscopic differences 
recorded in Tables I., II., and III., however, show numerous inversions even in the 
behaviour of the same line in different prominences, so that the increased brightness 
observed cannot always be due to this cause alone. 
Some of the lines are brightest at the base of the chromosphere, while others are 
brighter at greater elevations. As already explained (p. 586), lines which are brightest 
above the photosphere must be produced by vapours existing in layers concentric 
with, but detached from the photosphere. Those lines which become dimmer in 
passing outwards must owe their origin to vapours resting on the photosphere. 
In contrasting the spectrum of the prominences with the spectrum of the cusp, it 
should be borne in mind that the cusp in the African Photograph No. 22 does not 
represent the base of the chromosphere immediately beneath either of the metallic 
prominences considered in Table I. Still the cusp is not far from prominence No. 19, 
and it is fair to consider the base of the chromosphere homogeneous. If so, the 
prominences cannot be fed from the base of the chromosphere, since they contain 
different vapours. 
The preliminary discussion of individual substances has further abundantly shown 
that although some of the lines belonging to any particular metal may appear as dark 
lines in the solar spectrum on account of absorption by the chromosphere, other lines 
of the same substance are only represented among the dark lines because of absorption 
taking place elsewhere. This again is an indication of the stratification of the sun’s 
absorbing atmosphere, which, if it exists, must furnish a very strong argument in 
favour of the dissociation of metallic vapours at solar temperatures. In fact, the 
