1 6 LIGHT SCIENCE FOR LEISURE HOURS. 



resulting spectrum would be merely a very faint repro- 

 duction of the solar spectrum. 



Recently, however, the zodiacal light has been ana- 

 lysed by Angstrom, with a result altogether unexpected, 

 and at present almost unintelligible. Its spectrum 

 exhibits a bright line, and this bright line is the same 

 that is seen in the spectrum of the aurora borealis! 



How are we to understand this most surprising 

 result ? Remembering that the aurora is undoubtedly 

 a terrestrial light, whencesoever it derives its luminosity 

 in other words, that the electric discharges, however 

 excited, really take place in the upper regions of our 

 own atmosphere, while as certainly the zodiacal light is 

 an extra-terrestrial phenomenon the observed pheno- 

 menon becomes one of the most perplexing discoveries 

 ever made by man. That it will before long be inter- 

 preted we have no doubt whatever ; nor do we doubt 

 that the interpretation will involve the explanation of 

 a whole series of phenomena which have lately per- 

 plexed astronomers. Recalling the association between 

 auroras and terrestrial magnetism, and that between 

 terrestrial magnetism and the solar spots, and remem- 

 bering further that our physicists have recently de- 

 tected well-marked signs that the planets in their 

 courses influence the sun's atmosphere and generate his 

 spots in some manner as yet unexplained, we see that 

 the one fact wanting to explain Angstrom's discovery 

 is undoubtedly not an isolated fact, but must be asso- 

 ciated in the most intimate manner with a variety of 

 important cosmical relations. To speculate as to the 



