PHYSICS NINETEENTH CENT.SPECTROSCOPY. 499 



a sealed tube ; but by changing a little the conditions of the experi- 

 ment, Mr. Huggins found that only one line appeared in the nitrogen 

 spectrum, and this was coincident with the bright line of the nebulae. 

 There are, however, two other very conspicuous double lines in the 

 ordinary spectrum of nitrogen, which lines are altogether absent from 

 the nebulae spectra. How then, it may be asked, has Mr. Huggins 

 explained the absence of these two ? He found that when between the 

 eye and a spectroscope a certain thickness of neutral-tinted glass was 

 interposed, the result was that all the lines of the ordinary spectrum 

 of nitrogen ceased to be visible except this green line, which co- 

 incides with that of the nebulae. Similarly with regard to the lines of 

 the hydrogen spectrum, the intense blue line (corresponding with the 



FIG. 226. 



Fraunhofer line F) was the last to disappear. This explanation of the 

 disappearance of the weaker lines of a spectrum, attributes to some- 

 thing in the cosmical spaces traversed by the light a power of extin- 

 guishing indifferently the rays of light. The fact that this is quite 

 consistent with theoretical considerations that have been advanced on 

 other grounds increases the probability in its favour. The reader will 

 perceive that these single coincidences do not prove the existence of 

 nitrogen and of hydrogen in the nebulae, with the same certainty as 

 the 460 coincidences of the iron lines prove the existence of that 

 metal in the sun. There are, however, considerations derived by 

 analogy from the important part known to be played by the element 

 hydrogen in our own sun, which add weight to the theory of Mr. 

 Huggins. For the discoveries of Kirchhoff concerning the chemical 

 constitution of the sun were soon followed by a series of remarkable 

 discoveries in which the spectroscope opened out a new field of re- 

 search. In total eclipses of the sun, red-coloured prominences have 

 commonly been observed projecting from the sun's limb, and visible 

 only when the bright light of its disc is entirely intercepted by the moon. 

 Extending to a considerable distance beyond these is a fainter light, 

 called the corona, the nature of which has been long a subject of much 

 speculation. The corona and the prominences are roughly represented 



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