unless in the hands of the ablest mathematical physicists; and
Herschel violated their principles in other directions. But here his
speculations have attracted a great deal of attention, and, with
modifications, are accepted, at least as a working hypothesis, by a
fair number of people.

When Sir John Herschel had extended his father's researches into the
Southern Hemisphere he was also led to the belief that some nebulae
were a phosphorescent material spread through space like fog or mist.

Then his views were changed by the revelations due to the great
discoveries of Lord Rosse with his gigantic refractor,[22] when one
nebula after another was resolved into a cluster of minute stars. At
that time the opinion gained ground that with increase of telescopic
power this would prove to be the case with all nebulae.

In 1864 all doubt was dispelled by Huggins[23] in his first examination
of the spectrum of a nebula, and the subsequent extension of this
observation to other nebulae; thus providing a certain test which
increase in the size of telescopes could never have given. In 1864
Huggins found that all true nebulae give a spectrum of bright
lines. Three are due to hydrogen; two (discovered by Copeland) are
helium lines; others are unknown. Fifty-five lines have been
photographed in the spectrum of the Orion nebula. It seems to be
pretty certain that all true nebulae are gaseous, and show almost
exactly the same spectrum.

Other nebulae, and especially the white ones like that in Andromeda,
which have not yet been resolved into stars, show a continuous
spectrum; others are greenish and give no lines.