MITCHELL— THE FLASH SPECTRUM. 267 



have been obtained by the writer through the use of a colhmator 

 designed by Jewell. 



7n 1905 with a Rowland concave grating of 15,000 lines per inch, 

 Mitchell secured a photogrph of the flash spectrum in which he 

 measured 2,841 lines between A 3300 in the violet, and the D3 line 

 in the yellow. On account of the good definition, it was possible to 

 determine wave-lengths to 0.02 Angstroms correspondiog to an error 

 of measurement of 0.002 mm. 



The general conclusions from these measures were : 



1. The flash spectrum is a reversal of the Fraunhofer spectrum. 



2. The flash is not an instantaneous appearance, but the chromo- 

 spheric lines appear gradually. At the beginning of totality, those 

 of greatest elevation appear first, and at the end of totality remain 

 the last. The "reversing layer" which contains the majority of 

 the low-level lines is about 600 km. in height. 



3. Wave-lengths in the chromospheric and solar spectrum are 

 practically identical. 



4. The lines in the chromospheric spectrum differ greatly in in- 

 tensities from the lines in the solar spectrum. The chromospheric 

 spectrum shows the hydrogen series of lines and the helium lines 

 and corresponds to a spectral type earlier than the solar spectrum. 



5. The dififerences in intensities find a ready explanation in the 

 heights to which the vapors ascend. 



6. Especially prominent in the chromosphere are the enhanced 

 lines. 



In view of the very excellent work done recently at Mt. Wilson, 

 it has become of the utmost importance to have as accurate a knowl- 

 edge as possible of the heights of the various layers of the sun's 

 chromosphere above the photosphere. There is no other method of 

 determining these heights directly except from the photographs of 

 the flash spectrum at the time of a total eclipse taken without slit. 

 By knowing the angular diameters of sun and moon, it is easy to 

 calculate the height of the solar layers of atmosphere by measuring 

 the lengths of the cusps. On the photograph a spectrum line of 

 considerable length corresponds to a greater height of atmosphere, 

 while a shorter line belongs to a low-lying vapor. 



