150 



PROF. A. FOWLER, F.R.S., ON 



Sir Norman Loekyer. Lines which are intensified, or which 

 only appear, under spark conditions have been called " enhanced 

 lines," and it is to the study of such lines that much of the 

 progress of recent years has been due. 



Kirchhoffs famous experiment of 1859 on the reversal of 

 spectrum lines from bright to dark is of fundamental import- 

 ance in astronomy, because the spectrum of the Sun, and the 

 spectra of nearly all the stars, show dark lines on a bright 

 continuous background. The experiment proves that we can 

 identify the substances which produce such dark lines, just as 

 surely as if they were bright, by the process of matching them 

 by emission spectra artificially produced. 



In the spectrum of the Sun, which may be regarded as the 

 nearest star, Eowland has catalogued some 20,000 dark lines, 

 and the great majority of the more prominent have already been 

 matched by spectra produced in the laboratory, largely from 

 common substances such as hydrogen, sodium, iron, and calcium. 

 Observations of eclipses of the Sun have shown that these gases 

 and vapours exist chiefly in a shallow stratum, about 500 miles in 

 depth, which has been called the " reversing layer " or " Hash 

 stratum." Hydrogen, helium, and calcium are the chief con- 

 stituents of the overlying chromosphere, which has a depth of 

 about 5,000 miles. The corona, which is the most striking 

 feature of a solar eclipse, exhibits a few bright lines of at 

 present unknown origin, and has apparently nothing to do with 

 the dark lines of ordinary sunlight. Many of our chemical 

 elements have not yet been traced in the Sun, but reasonable 

 explanations of their lack of visible manifestation have been 

 adyanced, and there is no sufficient reason to suppose that the 

 composition of the Sun is materially different from that of the 

 Earth. 



Our present extensive knowledge of stellar spectra has been 

 made possible by the application of photographic methods of 

 obseryation. All stars are alike in the sense that they are 

 highly heated self-luminous bodies, but they are not all alike in 

 the character of the light which they emit. Thousands of 

 them resemble the Sun very closely, and what has been learned 

 about the Sun in more favourable circumstances is equally 

 applicable to stars of this class. It was early found that the 

 number of distinct varieties of stars was by no means large. 

 Father Secchi recognised four principal types of stellar spectra, 

 which he numbered from I to IV, beginning with white stars 

 and ending with red ones. It was not loner before this classi- 

 fication came to be regarded as something more than a mere 



