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and C of Brewster’s spectrum ; it is not easy to determine 
accurately with which of the dark lines this yellow line is 
coincident. 
Strontian gives six red, one orange, and one blue line. 
Calcium and its salts, a bright green hne, an intensely brig’ht 
orange line, and the paler intermediate bands. Barium gives 
well-defined green lines, some yellow lines, varying in intensity, 
an orange hne, and indications of red. 
Such are the general characteristics of the spectral images 
produced. Kirchoff and Bunsen say, in arguing upon these 
lines, and the hypothesis of their representing the solar dark 
lines : “ It was proved from theoretical considerations, that the 
spectrum of an incandescent gas becomes reversed (that is, that 
the bright ones become changed into dark ones) when a source 
of light of sufficient intensity, giving a continuous spectrum, is 
placed behind the luminous gas. From this we may conclude 
that the solar spectrum, with its dark lines, is nothing else than 
the reverse of the spectrum, which the sun’s atmosphere alone 
would produce. Hence in order to effect the chemical analysis 
of the solar atmosphere, all that we require is to discover those 
substances which, when brought into the flame, produce bright 
lines coinciding with the dark ones of the solar spectrum.” 
The coincidence of the soda hne with dark line D is striking; 
one of the yellow lines produced by barium appears to 
correspond with it, and it must not be forgotten that Angstrom 
proved the yellow lines produced by other bodies to be 
coincident with D.* The red hne of the potassium spectrum 
is coincident with the dark space A. The other lines are not 
so satisfactorily determined : where there are such a number 
of clarh lines, there is no difficulty in assigning to one of the 
bright ones a place amongst them ; but it requires the utmost 
care to determine the exact coincidence of a bright with a dark 
space. 
The next step in the process of the investigation instructs us 
in the fact, that the vapours producing’ those coloured flames are 
opaque to their own rays. That is to say, if we produce a yellow 
soda-flame, and from it obtain a spectrum showing the peculiar 
soda lines in their bright yellow colour, and then impregnate the 
air with some soda vapour, by volatilizing soda between the flame 
and the spectrum, the bright yelloiv line becomes at once a black 
* While those pages have been passing through the press, a communica- 
tion has been made to the Parisian J ournal ( The Cosmos), by M. Morren, 
urging the necessity of caution in making deductions, because, as stated in 
this paper, the yellow line produced by Sodium is equally producible by 
other bodies. Professor Franldand and Dr. Tyndall, in the Philosophical 
Magazine for December, call attention to the influences of variation of 
temperature in altering the character of some of the lines. 
