J une ▁ 8 , ▁ 1 9 3 7 ), ▁this ▁time ▁as ▁the ▁scientific ▁leader ▁of ▁a ▁National ▁Geographic ▁Society ▁Ex ped ition . ▁An ▁article ▁entitled ▁" N ature ' s ▁Most ▁D ram atic ▁Spect acle " ▁by ▁Mitchell ▁appeared ▁in ▁the ▁September ▁ 1 9 3 8 ▁edition ▁of ▁National ▁Geographic ▁Magazine . ▁These ▁ten ▁exped itions ▁allowed ▁him ▁to ▁write ▁E cli ps es ▁of ▁the ▁Sun , ▁summar izing ▁his ▁work ▁on ▁solar ▁flash ▁spect ra , ▁first ▁published ▁in ▁ 1 9 2 3 ▁and ▁produced ▁through ▁five ▁ed itions ▁( 5 th ▁edition , ▁ 1 9 5 1 ). ▁On ▁the ▁ 1 9 1 8 ▁Oregon ▁and ▁the ▁ 1 9 2 5 ▁Connecticut ▁e cli ps es , ▁Mitchell ▁was ▁accompanied ▁by ▁the ▁artist ▁Howard ▁Russell ▁Butler ▁( 1 8 5 6 – 1 9 3 4 ), ▁whose ▁paint ings ▁of ▁tot ality ▁g rac ed ▁the ▁old ▁Hay den ▁Planet arium ▁of ▁the ▁American ▁Museum ▁of ▁Natural ▁History ▁for ▁many ▁years . ▁ ▁Par alla x ▁work ▁and ▁Le ander ▁McC orm ick ▁Observ atory ▁Mitchell ▁went ▁back ▁to ▁Y erk es ▁for ▁the ▁sum mers ▁of ▁ 1 9 0 9 , ▁ 1 9 1 0 ▁and ▁ 1 9 1 1 ▁and ▁then ▁returned ▁for ▁a ▁fifteen - month ▁s abb at ical ▁in ▁ 1 9 1 2 ▁and ▁ 1 9 1 3 . ▁Frank ▁Schles inger ▁first ▁demonstrated ▁the ▁technique ▁of ▁determ ining