PROGRESS OF METEORIC ASTRONOMY IN AMERICA. 301 
To these general rules there are some important exceptions 
or modifications. 
If the temperature of certain vapors or gases be raised to 
a high degree the number and the appearance of the colored 
band or of the bright lines change rapidly, though not uni¬ 
formly, and some investigators have asserted that if the 
temperature be raised to something over 4,500° Fah. the 
spectrum will become practically continuous. Similar 
changes in the phenomena are observed if a gas, like hydro¬ 
gen, is rendered luminous by the electric spark and then 
subjected to varying pressures. With a pressure amounting 
to one-twentieth of an inch of mercury the spectrum is dis¬ 
continuous and consists of several groups of bright lines in 
the green. As the pressure is gradually increased there ap¬ 
pears a temporary spectrum of bands, then a spectrum of 
three lines, afterwards a more permanent and complete 
spectrum of bands, and finally, under a pressure of 52 inches 
of mercury, a complete and pure continuous spectrum. 
The spectra of comets, which have been obtained by care¬ 
ful and experienced observers, present a large number of 
variations and combinations, ranging from one or more 
faint bands with indistinct or fluted borders against a color¬ 
less background to a faint continuous spectrum with bands 
or lines of a greater or less degree of brightness and defini¬ 
tion. 
The most obvious interpretation of the spectroscopic ob¬ 
servations of comets is that the bands and lines are the true 
spectra of a gaseous body, varying through a wide range 
under the effect of changing pressure and temperature, super¬ 
imposed upon the faint continuous spectrum derived from 
the sunlight reflected from the nucleus or other parts of the 
comet. 
Such is the information derived from the spectroscope. 
In the vast number of observations of comets, made for 
the determination of their positions or their physical pecu¬ 
liarities, it has sometimes been noted that the comet passed 
