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are most conspicuous in the chromosphere light, do actually cause 
uneommonly great anomalous dispersion — a wide field for experi- 
mental research which to explore the first steps have scarcely been 
taken as yet. 
On the other hand, as regards the self-luminosity of gases, LOCK YER's 
ingenious experimental method of long and short lines affords us 
an invaluable help to investigate what is the influence of the tempe- 
rature (and the density?) of the radiating substance on the emission 
spectrum. So it seems possible to make out by experiment whether it 
is radiation or refraction to which the different chromosphere lines 
are most probably due. 
This decision ought, of course, to be founded on a most accurate 
knowledge of the character which each of the spectral lines of the 
solar atmosphere exhibits in different circumstances. The coming total 
solar eclipses offer a good opportunity to observe the chromosphere 
spectrum minutely, little disturbed by the dazzling light of the photo- 
sphere. Especially it is to be hoped that some good spectrograms 
will be obtained with high dispersion apparatus. 
Let us now consider from the point of view of anomalous disper- 
sion the well-known “reversing-layer’’, which in total eclipses causes 
the so-called “flash”. We have seen before that the theory of dis- 
persion assigns anomalous dispersion to all waves whose periods lie 
near each characteristic vibration-period of a substance; but the 
amount of the anomalous dispersion may be slight. In such a case 
the arrows, in an experiment similar so that described for sodium- 
light, would be short and narrow, but, for all that, of great intensity. 
If, therefore, such substances exist in the solar atmosphere even 
at great distances from the photosphere, with irregularities in density 
similar to those assumed for sodium, hydrogen etc., the anomalous 
refraction will betray the presence of those substances merely in the 
immediate vicinity of the edge of the sun’s disc, and only during a 
few seconds at the beginning and the end of the totality of an eclipse. 
This view of the subject makes it a matter of course that the lines 
of the flash should be very bright; for properly speaking, it is not 
chiefly the radiation, emitted by the vapours, that we observe, but 
photosphere light of pretty much the same wavelength. Nor is it 
necessary that the gases in those places should be of extraordinary 
great density, or that their presence should be restricted to a thin 
reversing layer — one of the most mysterious things the solar theory 
has led up to and which they have tried to escape in various ways. 
The light of the chromosphere- and of the flash-lines may be sym- 
