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size of telescopes could never have given.
In 1864 Huggins found that all true nebu-
lae give a spectrum of bright lines. Three
are due to hydrogen; two (discovered by
Copeland) are helium lines; others are un-
known. Fifty-five lines have been photographed
in the spectrum of the Orion nebula. It
seems to be pretty certain that all true neb-
ulae are gaseous, and show almost exactly
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