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"A spectrum of sunlight is crossed by dark lines, many hundreds of whcih have been counted and accurately mapped. The more conspicious of these dark lines are distinguisged by letters of the alphabet. A few of these dark lines in the solar spectrum are due to absorption in the earth's atmosphere, but by far the greater number originate in the selective absorption of the solar atmosphere itself." — Avery, 1895

Visible spectrum

"A spectrum of sunlight is crossed by dark lines, many hundreds of whcih have been counted and accurately…