300, 3 oi] Spectrum Analysis 38 



atmosphere, the majority cannot be, and are therefor- 

 caused by gases in the atmosphere of the sun. 



For example, the metal sodium when vaporised gives a 

 spectrum characterised by two nearly coincident bright 

 lines in the yellow part of the spectrum ; these agree in 

 position with a pair of dark lines (known as D) in the 

 spectrum of the sun (see fig. 97); Kirchhoff inferred there- 

 fore that the atmosphere of the sun contains sodium. By 

 comparison of the dark lines in the spectrum of the sun 

 with the bright lines in the spectra of metals and other sub- 

 stances, their presence or absence in the solar atmosphere 

 can accordingly be ascertained. In the case of iron which 

 has an extremely complicated spectrum Kirchhoff suc- 

 ceeded in identifying 60 lines (since increased to more 

 than 2,000) in its spectrum with dark lines in the spectrum 

 of the sun. Some half-dozen other known elements were 

 also identified by Kirchhoff in the sun. 



The inquiry into solar chemistry thus started has since 

 been prosecuted with great zeal. Improved methods and 

 increased care have led to the construction of a series of 

 maps of the solar spectrum, beginning with Kirchhoffs own, 

 published in 1861-62, of constantly increasing complexity 

 and accuracy. Knowledge of the spectra of the metals, has 

 also been greatly extended. At the present time between 

 30 and 40 elements have been identified in the sun, the 

 most interesting besides those already mentioned being 

 hydrogen, calcium, magnesium, and carbon. 



The first spectroscopic work on the sun dealt only with 

 the light received from the sun as a whole, but it was soon 

 seen that by throwing an image of the sun on to the slit 

 of the spectroscope by means of a telescope the spectrum 

 of a particular part of the sun's surface, such as a spot or 

 a facula, could be obtained; and an immense number of 

 observations of this character have been made. 



301. Observations of total eclipses of the sun have shewn 

 that the bright surface of the sun as we ordinarily see it 

 is not the whole, but that outside this there is an envelope 

 of some kind too faint to be seen ordinarily but becoming 

 visible when the intense light of the sun itself is cut off 

 by the moon. A white halo of considerable extent round 

 the eclipsed sun, now called the corona, is referred to by 



