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the solar atmosphere, the asymmetry, and with it the displacement, 
become imperceptible. 
A displacement of the majority of the Fraunhofer lines towards 
the greater wave-lengths, amounting to a few thousandth parts of an 
Angstrom unit, was discovered by Jewell as early as 1896. But 
there were numerous exceptions: lines showing no displacement, or 
a displacement towards the violet (with respect to their positions in 
the emission spectrum produced in the laboratory). 
Recently M. M. Fabry and Buisson *) applied their tine, extremely 
accurate interferential method in comparing iron lines of the arc- 
spectrum with those of the solar spectrum. Only narrow lines could 
be studied in that way. Their results confirmed those obtained by 
Jewell: in general displacement to the red, but many exceptions. 
The latter, however, appeared to bear exclusively upon lines which 
under ordinary circumstances are not very sharp in the arc-spectrum, 
and are widened asymmetrically when the current is increased. 
With such lines the localisation in the emission spectrum had perhaps 
not been so accurate as could be desired. Fabry and Buisson suc¬ 
ceeded 2 ) in making these dubious lines perfectly sharp, by producing 
the iron-arc in vacuo. On comparing the solar spectrum with the 
arc-sgectrum thus obtained, they found, that the exceptions had 
disappeared: all the lines examined were shifted to the red in the 
Fraunhofer spectrum. The displacements ranged from 0,005 to 0,010 
Angstr. units with lines whose width varied between 0,07 and 0,16 
units (intensity 1 to 8 according to Rowland’s scale). 
At the beginning of their interesting communications Fabry and 
Buisson remark: “Le deplacement des raies permet (done) de mesurer 
les variations de pression. En comparant les raies du spectre solaire 
avee les raies correspondantes de l’arc a la pression atmos- 
pherique, on a un moyen d’evaluer la pression de la couclie ren- 
versante, si toutefois aucun autre phenomene n’intervient.” Supposing 
that there were no other disturbing phenomena except those which 
they had excluded in their experiments, they calculate that the 
average pressure in the reversing layer must be from 5 to 6 atmospheres. 
We hold the opinion that this valuation is certainly too high. Indeed, 
the observed displacements cannot have been exclusively produced 
by pressure, because refraction is sure to bring about line-shifts in 
the same direction. And if perhaps future research might induce one 
to think it p robable, that at the same level of the sun the pressure 
Fabry et Buisson, Comptes rendus, 15 mars et 29 mars 1909. 
s ) Fabry et Buisson, Comptes rendus, 10 mai 1909. 
