F. W. Very— A Cosmic Cycle. 53 



the passage of rajs of short wave-length, has displaced the 

 solar maximum towards the infra-red. 



Although the temperature of the deeper portions of a star 

 may increase as it contracts, its effective surface-temperature, 

 as measured by its radiation, progressively diminishes. For 

 proof of this in the case of our sun, we have only to refer to 

 geologic evidence. 



Tropical forms of life occur in higher latitudes as we go 

 back through the geologic ages. " The general climate cannot 

 be sensibly affected by conducted heat (from the earth's inte- 

 rior) at any time more than 10,000 years after the commence- 

 ment of superficial solidification." * Retention of radiation 

 may be altered temporarily by slight modifications of the 

 earth's atmosphere produced by volcanic eruptions or by 

 meteoric accessions ; but the continual lowering of terrestrial 

 temperature is evidence of a steady diminution of solar 

 efficiency, partly because the angle subtended by the solar 

 sphere has grown smaller, and partly because the absorption of 

 the solar atmosphere has become greater. 



The safest criterion of a progressively deepening photo- 

 spheric surface and increasing stellar atmosphere is the cessa- 

 tion of the shorter waves as a whole. If we choose the breadth 

 of a line in the spectrum as a signature of pressure, we shall 

 get a different estimate of the density of the stellar atmosphere 

 according to the substance selected. Thus great breadth of the 

 helium lines would accuse some of the Orion stars of having 

 the densest atmospheres — hydrogen would speak for the Sirian 

 stars to the same effect — calcium for the solar stars, and so on. 



/Spectral Variations connected ivith Changes of Density. 



As helium glows in the solar chromosphere without mani- 

 festing its presence by any dark Fraunhofer lines, we are not 

 permitted to infer the absence of a substance from sun or star 

 by the failure of its spectral lines. Helium exists about the 

 sun as an elevated and rarefied shell. The seldom varying 

 breadth of its chromospheric lines from the limb up to a 

 height of about one thousand miles, and the smaller distortion 

 of the lines, show that, unlike hydrogen whose chromospheric 

 lines are often broadened and distorted, the helium atmosphere 

 is comparatively quiescent and probably greatly rarefied. f 



* W. Thomson (Lord Kelvin), Trans. E. Soc. Edinburgh, vol. xxiii, pt. 1, p. 164, 

 1862. 



fin Frost's translation of Scheiner's "Astronomical Spectroscopy" (p 189, 

 1894), it is said that "the D 3 line presents an entirely different appearance [from 

 that of the hydrogen lines in the chromospheric spectrum], for while following the 

 shape of the prominence in general, it never broadens at the limb, but on the contrary 

 grows narrower, and possibly it does not touch the limb at all. This would indi- 



