Joseph von Fraunhofer · Spectral lines
What are the dark lines inside sunlight?
That time, that place
Orphaned at eleven, Fraunhofer was apprenticed to a mirror-maker. At fourteen the workshop collapsed and buried him; the story goes that the prince-elector, watching the rescue, took notice and supported him afterwards.
His work was making telescope lenses. Good ones require knowing very precisely how much the glass bends light — and that amount differs for every colour.
So he needed light of a single colour.
Why this question
He was using the yellow of a sodium flame as his reference. For comparison he let sunlight in through a narrow slit and put it through a prism.
Inside the band of colours were thin dark lines. Not one or two.
He began counting. He recorded 574, labelled the strong ones A through K, measured their positions precisely — and confirmed that however often he looked, the lines sat in exactly the same places.
What was found
He did not know why the lines were there. His purpose was served — he had the reference lines his lenses needed. He was a maker of instruments, not a builder of theories.
Thirty-three years later Kirchhoff and Bunsen gave the answer. Each element emits only certain colours and, conversely, absorbs only those. Sunlight passing through the sun's outer layers is robbed of exactly those colours, and the gaps remain as dark lines.
So read the positions of the lines and you know which elements the sun contains. A way to know a composition without going there.
In 1835 the philosopher Comte is said to have offered the chemical composition of the stars as his example of what humans can never know. When he chose that example, the answer had been sitting in Fraunhofer's notebook for twenty years.
What the sun and stars are made of belonged to the unknowable
Precise measurements showing the dark lines always fall in exactly the same places
Thirty-three years later the lines proved to be elemental fingerprints, and astronomy became a science that reads composition from light alone