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1915]                       ON  THE WIDENING OF SPECTRUM  LINES                          299
great difference whether two surfaces of a Bunsen soda flame (front and back) are in action or only one. If the supply of soda to each be insufficient to cause dilatation, the multiplication of flames in line (3 or 4) has no important effect either upon the brightness or the width of the lines. Actual measures, in which no high accuracy is needed, would here be of service.
The observations referred to led me many years ago to make a very rough comparison between the light actually obtained from a nearly undilated soda line and that of the corresponding part of the spectrum from a black body at the same temperature as the flame. I quote it here rather as a suggestion to be developed than as having much value in itself. Doubtless, better data are now available.
How does the intrinsic brightness of a just undilated soda flame compare with the total brightness of a black body at the temperature of the flame ? As a source of light Violle's standard, viz. one sq. cm. of just melting platinum, is equal to about 20 candles. The candle presents about 2 sq. cm. of area, so that the radiating platinum is about 40 times as bright. Now platinum is not a black body and the Bunsen flame is a good deal hotter than the melting metal. I estimated (and perhaps under estimated) that a factor of 5 might therefore be introduced, making the black body at flame temperature 200 times as bright as the candle.
To compare with a candle a soda flame of which the D-lines were just beginning to dilate, I reflected the former nearly perpendicularly from a single glass surface. The soda flame seemed about half as bright. At this rate the
intrinsic brightness of the flame was ^ x ^ =  of that of the candle, and
2i      jO     o\)
accordingly TTT-PTTT/C of that of the black body.
JLV_f j\J\J\J
The black body gives a continuous spectrum. What would its brightness be when cut down to the narrow regions occupied by the D-lines ? According to Abney's measures the brightness of that part of sunlight which lies between
the D's would be about ^r^ of the whole.    We may perhaps estimate the
region actually covered by the soda lines as ~-= of this.    At this rate we
should get
^ x
25    250    6250'
as the fraction of the whole radiation of the black body which has the wavelengths of the soda lines. The actual brightness of a soda flame is thus.of the same order of magnitude as that calculated for a black body when its spectrum is cut down tq that of the flame, and we may infer that the light of a powerful soda flame is due much more to the widening of the spectrum lines than to an increased brightness of their central parts. wcnm t rrinke rt<*