54 F. W. Very — A Cosmic Cycle. 



Coronium floats at a still higher level than helium, and it 

 likewise appears to lack representation among the Fraunhofer 

 lines, the old supposed identification having proved erroneous. 

 We may presume that if any other ingredient of the sun's 

 reversing layer were removed to such an altitude as to be 

 greatly expanded, its dark Fraunhofer lines would disappear 

 from the spectrum, unless one or more of them might linger 

 by virtue of extraordinary intensity. 



In a different category belong those elements of great den- 

 sity which at a certain stage of condensation may be deeply 

 buried and thus fail to appear, but which may afterwards be 

 brought to the surface. In this case are those metals whose 

 lines first begin to show feebly in the spectra of the Sirian 

 stars. 



Nevertheless, although the apparent absence of an element 

 may be explained in some such way, these explanations do not 

 rule out the possibility that the apparent absence or weakening 

 of an element in a star may be due to an actual elimination. 

 Still less can they be invoked either to prove or disprove the 

 possibility of such a general breaking down of atomic struc- 

 tures as I have suggested. 



Pressure produces great spectral changes. " The whole 

 character of the spectrum of iron, for instance, is changed 

 when we pass from the Fraunhofer lines to those seen among 

 the spot and prominence lines ; a complex spectrum is turned 

 into a simple one, the feeble lines are exalted, the stronger 

 ones suppressed almost altogether."* We know that a very 

 wide range of pressure must exist in different stars and at dif- 

 ferent stages of condensation in the same star. A first effect 

 of condensation is an increasing heterogeneity and a separation 

 of ingredients into successive shells of varying composition, 



cate that helium has a less density near the limb than in the higher strata of the 

 solar atmosphere." With greater dispersion, however, this constriction at the 

 limb is not seen, but the line appears hazy and of uniform breadth through the 

 greater part of its length, becoming fainter and narrower at its upper extremity. 

 With still greater dispersion, according to Hale, the helium lines broaden in the 

 lower part of the chromosphere much as the hydrogen lines do. These distinc- 

 tions, therefore, depend entirely upon the brilliancy of the background of continu- 

 ous spectrum on which the bright lines are superimposed. The distribution of 

 luminosity on either side of the center, of the line is not the same in these two 

 cases. The hazy wings of the hydrogen lines diminish more gradually in inten- 

 sity on receding from the center. The broad wings of helium are hard to see 

 near the limb, consequently helium falls off very suddenly in brightness on either 

 side of the center of a line, and it is probable that in this respect the curve of 

 emission for a helium line (coordinates = wave-length and intensity) resembles the 

 curve of absorption very closely, and that this is a principal cause for the feeble- 

 ness of the helium absorption lines. But it cannot be the only reason, since 

 helium appears dark in stellar spectra, and the cause suggested in the text may 

 be an additional one for its non-appearance in the solar spectrum. 

 *J. Norman Lockyer, The Chemistry of the Sun, p. 253. 



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