ADDRESS. 27 



But it is not merely the chemistry of the heavenly bodies on which 

 light is thrown by the specti'oscope ; their physical structure and evolu- 

 tional history are also illuminated by this wonderful instrument of 

 research. 



It used to be supposed that the sun was a dark body enveloped in 

 a luminous atmosphere. The reverse now appears to be the truth. The 

 body of the sun, or photosphere, is intensely brilliant ; round it lies the 

 solar atmosphere of comparatively cool gases, which cause the dark lines 

 in the spectrum; thirdly, a chromosphere, — a sphere principally of 

 hydrogen, jets of which are said sometimes to reach to a height of 

 100,000 miles or more, into the outer coating or corona, the nature of 

 which is still very doubtful. 



Formerly the red flames which represent the higher regions of the 

 chromosphere could be seen only on the rare occasions of a total solar 

 eclipse. Janssen and Lockyer, by the application of the spectroscope, 

 have enabled us to study this region of the sun at all times. 



It is, moreover, obvious that the powerful engine of investigation 

 afiForded us by the spectroscope is by no means confined to the substances 

 which form part of our system. The incandescent body can thus be 

 examined, no matter how great its distance, so long only as the light is 

 strong enough. That this method was theoretically applicable to the 

 light of the stars was indeed obvious, but the practical difficulties were 

 very great. Sirius, the brightest of all, is, in round numbers, a 

 hundred millions of millions of miles from us ; and, though as big as sixty 

 of our suns, his light when it reaches us, after a journey of sixteen 

 years, is at most one two-thousand-millionth part as bright. Nevertheless 

 as long ago as 1815 Fraunhofer recognised the fixed lines in the light 

 of four of the stars, and in 1863 Miller and Huggins in our own 

 country, and Rutherford in America, succeeded in determining the dark 

 lines in the spectrum of some of the brighter stars, thus showing that 

 these beautiful and mysterious lights contain many of the material 

 substances with which we are familiar. In Aldebaran, for instance, we 

 may infer the presence of hydrogen, sodium, magnesium, iron, calcium, 

 tellurium, antimony, bismuth, and mercury ; some of which are not yet 

 known to occur in the sun. As might have been expected the composi- 

 tion of the stars is not uniform, and it would appear that they may 

 be arranged in a few well-marked classes, indicating difierences of 

 temperature or, in other words, of age. Some recent photographic spectra 

 of stars obtained by Huggins go very far to justify this view. 



Thus we can make the stars teach us their own composition with 

 light which started from its source before we were born — light older 

 than our Association itself. 



Until 1864, the true nature of the unresolved nebulae was a matter of 

 doubt. In that year, however, Huggins turned his spectroscope on to 

 a nebula, and made the unexpected discovery that the spectra of some 

 of these bodies are discontinuous — that is to say, consist of bright lines 



