THE FLASH SPECTRUM. 



By S. a. MITCHELL. 

 (Read April 25, iQiQ-) 



The total eclipse of 1918 saw the completion of fifty years since 

 the spectroscope was first used at the eclipse of 1868, visible in 

 far-off India. If all the minutes of totality during this half century 

 of eclipses were added together, they would amount to less than 

 one brief hour. It is safe to say, however, that no other branch of 

 astronomy has shown the remarkable value of the new instrument 

 of research as has the work of eclipse spectroscopy. 



In 1868, Janssen discovered that the prominences gave a bright 

 line spectrum thus proving that they are gaseous in nature. As is 

 well known, he and also Lockyer independently, showed how to view 

 the prominences without an eclipse. 



The year 1869 brought with it the discovery of helium in the 

 sun. Since the urgency of the war has resulted in having helium 

 supplied in such large quantities and at such a cheap price that it 

 may be used in balloons and airships, it is almost a surprise to think 

 that helium was not discovered by Ramsay tmtil the year 1895. 



The eclipse of 1870 visible in Spain is noted for the discovery of 

 the flash spectrum. The dark lines in the solar spectrum, the Fraun- 

 hofer lines are caused by the absorption of the light of the photo- 

 sphere by a thin layer of cooler gases, the so-called reversing layer. 

 This layer is cool only in contrast with the very hot photosphere. 

 As the moon gradually covers the sun at the time of an eclipse the 

 dark line spectrum persists so long as there is even the slightest 

 trace of the photosphere visible. At the instant when the photo- 

 sphere is entirely covered up, the bright photospheric background of 

 the Fraunhofer spectrum is removed, and the lines of the spectrum 

 now appear as bright lines on a black background, where before 

 there had been dark lines on a bright background. This change 

 coming with totality was foretold by Young of Princeton before the 



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