vii.] SOLAR ELEMENTS. 93 



temperature of the electric arc instead of the spectrum which 

 was produced at the temperature of the electric spark. 



The result of the combined attack of KirchhofF, Angstrom, 

 and Thalen is shown in the accompanying table : 

 Elements present in the Sun. 



Kirchhoff. Angstrom and Thalen. 



Sodium. Sodium. 



Iron. Iron. 



Calcium. Calcium. 



Magnesium. Magnesium. 



Nickel. Nickel. 



Barium. 

 Copper. 

 Zinc. 



Chromium. 



Cobalt. 



Hydrogen. 



Manganese. 



Titanium. 



So far then for that mode of observing the sun which consists 

 in comparing the general light of the light-source with the 

 general light of the sun. 



This introduces an important consideration. When we have 

 a light-source placed in front of the slit of the spectroscope it is 

 perfectly clear that light from all portions of the light-source 

 must illuminate the slit. Similarly, if we content ourselves by 

 pointing the spectroscope to the sun, or to a cloud illuminated 

 by the sun, it is perfectly obvious that the light from all parts 

 of the sun must enter all parts of the slit, and if there is any 

 localisation of phenomena on the sun this will be lost to a 

 greater or less extent in the spectroscopic record. Such 

 localisations there undoubtedly are, as may be seen with a 

 telescope of very moderate power, and a brief account of what 

 these are, and how they may be effectively got at by the spectro- 

 scope, may be conveniently given in the next chapter. 



