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Physics. — “The lines H and K in the spectrum of the various 
parts of the solar disk”. By Prof. W. H. Jumus. 
§ 1. Causes of line-shifts. 
There are at present four causes known, by which bright or dark 
lines of the solar spectrum may be displaced with respect to the 
position of the corresponding emission lines, as observed in labora- 
tory experiments: radial motion, pressure, magnetic fields, anomalous 
dispersion. Which of these causes is likely to be the effective one, 
in each special case, can only be decided by comparing from a 
physical point of view the possibility of the conclusions to which 
the different suppositions lead us. If, e.g. we try to explain, by each 
of the four principles, the strong line-displacements often observed 
in the spectrum of prominences, it soon appears that both the second 
and the third principle are unable to give satisfactory solutions of 
the problem, so that, on further research, we shall only have to 
choose between the first and the fourth. On the other hand, the 
general displacement of the Fraunhofer lines toward the red, 
increasing from the centre to the limb of the solar disk, can be 
accounted for neither by Doppimr’s principle, nor by the ZEEMAN 
effect; so the interpretation of the phenomenon as a pressure effect 
has to be compared, in its consequences, with the interpretation on 
the basis of anomalous dispersion. 
In a few cases there is scarcely any reason for doubt about the 
origin of certain line-displacements. Nobody will hesitate to ascribe 
the systematic differences between the spectra of the east and west 
limb of the sun to motion in the line of sight; nor question the 
magnetic origin of doublets and triplets in the spot-spectrum, which 
show the polarisation phenomena characteristic of the Zeeman effect. 
But such cases, where even at first sight only one explanation 
seems possible, are rare. It would be rash, e.g. to make magnetic 
forces at once responsible for the total widening of spot-lines, while 
other causes, that also produce widening, are known. As a rule, 
various influences co-operate; then the probably effectual cause of a 
solar phenomenon will only be brought out as such indirectly, by 
exclusion, that is, after other explanations have entangled one in 
ideas, clashing with general physical laws. And the remaining 
explanation will of course be the more probable, the better it joins 
some theory, already giving a coherent view of many other solar 
phenomena. 
§ 2. Phenomena exhibited by the calcium lines H and K. 
A remarkable case of systematic displacements, occurring with 
