( 1273 ) 
Besides, the question arises, what can.be the nature of the forces, 
giving such velocities to the co-existing rising and falling currents, 
and acting especially on ca/ccum, not, or at least in a much lesser 
degree, on other gases? Some additional hypothesis is badly wanted 
here. 
There are more difficulties, that one cannot avoid without intro- 
ducing special hypotheses. The widening of A, towards the limb is 
explained by the continuous increase of the depth of the layer of 
radiating calcium vapour in the line of sight on approaching the 
limb. It is supposed that a sensible part of the beam of calcium- 
lieht, reaching the observer, has been able to travel a distance of 
16000—62000 kilometers *) in a nearly straight line through a layer 
of the selectively absorbing solar gases, in which the average pres- 
sure is evaluated at one (terrestrial) atmosphere *). This conception 
seems to be opposed to the generally accepted theory of scattering 
and absorption of light. Moreover, one would expect, on the basis 
of the same explanation, to find the absolute brightness of A, in- 
creasing in passing from the centre to the limb. This, however, does 
not come true. Only in comparison with the neighbouring parts of 
the spectrum, A, increases in importance, but its absolute brightness 
decreases decidedly. In order to obtain spectrograms of nearly equal 
photographie density, Sr. JonN had to make the exposures 4 to 5 
times longer at the limb than at the centre. The current explanation 
of the phenomena therefore requires the indication of an additional 
agent or process, by which the radiation of the chromosphere, 
although supposed to increase in passing from the centre to the 
limb, appears to decrease. One might e.g. assume the existence of 
a medium, surrounding the chromosphere in a rather thin layer, 
and having the property of absorbing all kinds of light in a certain 
degree. 
Similar additional hypotheses need not be introduced, if we explain 
the phenomena, exhibited by the calcium lines in the spectrum of 
the various parts of the solar disk, by means of the theory of the 
propagation of light through extensive masses of gas. 
1) St. Jonn, Lc 
2) Sr. Joun, l.c 
p. 66. 
p. 43. 
wij 
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