A great deal has to be done by the chemist before the astronomer can
be on sure ground in drawing conclusions from certain portions of his
spectroscopic evidence.

The light of the nebulas is remarkably actinic, so that photography
has a specially fine field in revealing details imperceptible in the
telescope. In 1885 the brothers Henry photographed, round the star
Maia in the Pleiades, a spiral nebula 3' long, as bright on the plate
as that star itself, but quite invisible in the telescope; and an
exposure of four hours revealed other new nebula in the same
district. That painstaking and most careful observer, Barnard, with
10-1/4 hours' exposure, extended this nebulosity for several degrees,
and discovered to the north of the Pleiades a huge diffuse nebulosity,
in a region almost destitute of stars. By establishing a 10-inch
instrument at an altitude of 6,000 feet, Barnard has revealed the wide
distribution of nebular matter in the constellation Scorpio over a
space of 4 degrees or 5 degrees square. Barnard asserts that the "nebular
hypothesis" would have been killed at its birth by a knowledge of
these photographs. Later he has used still more powerful instruments,
and extended his discoveries.

The association of stars with planetary nebulae, and the distribution
of nebulae in the heavens, especially in relation to the Milky Way, are
striking facts, which will certainly bear fruit when the time arrives
for discarding vague speculations, and learning to read the true
physical structure and history of the starry universe.

_Stellar Spectra._--When the spectroscope was first available for
stellar research, the leaders in this branch of astronomy were Huggins