214 THE AMERICAN MUSEUM JOURNAL 
particularly in case of Comet a 1910 in the tail gives evidence of solar forces 
the curved, fanlike form of the matter repelling them. 
The dark-line nebula in the constellation Virgo. Photographed with 
the 40-inch reflector, April 18, 1911. An object of unusual interest on 
account of sts rapid motion — 600 miles per second in the line of sight — 
and also because it is the first nebula for which rotation about an axis 
and proper-motion have been observed 
The ‘‘dumb-bell’’ nebula in the constellation Vulpecula. Photo- 
graphed with the 40-inch reflector of the Lowell Observatory 
Both slit and slit- 
less prismatic photo- 
graphs of Halley’s, 
Gale’s, Brook’s and 
other comets of recent 
years, segregate and 
make known the va- 
rious constituents. In 
this analysis the prism 
differentiates the con- 
stitution of the head 
and tail, and even the 
substances composing 
different streamers in 
the tail can be iden- 
tified. In general, 
comet spectra are 
quite similar; they 
show the existence, 
usually, of cyanogen, 
sodium and_ hydro- 
carbons in the head 
while carbon mon- 
oxide and solid sun- 
lit particles produce 
mainly comet tails. 
The spectrograph 
serves also as a ba- 
rometer which enables 
one to read many of 
the existing conditions 
of planetary atmos- 
pheres. It tells us 
that Jupiter, Saturn, 
Uranus and Neptune 
are surrounded’ by 
very dense atmos- 
pheres and that the 
densities of these in- 
crease with the dis- 
tances of the planets 
from thesun. It was 
by this means that 
evidence has _ been 
secured at the Lowell 
