76 CELESTIAL SPECTROSCOPY. 
been regarded as single, and their supposed coincidence with terrestrial 
lines falls to the ground. For this reason many of the early conclusions 
based on observation as good as it was possible to make at the time 
with the less powerful spectroscopes then in use, may not be found to be 
maintained under the much greater resolving power of modern instru- 
ments. 
Spectroscopic Problems.—The spectroscope has failed as yet to inter- 
pret for us the remarkable spectrum of the aurora borealis. Undoubt- 
edly in this phenomenon portions of our atmosphere are lighted up by 
electric discharges; we should expect, therefore, to recognize the spectra 
of the gases known to be present in it. As yet we have not been able 
to obtain similar spectra from these gases artificially, and especially we 
do not know the origin of the principal line in the green, which often 
appears alone, and may have, therefore, an origin independent of that of 
the other lines. Recently the suggestion has been made that the aurora 
is a phenomenon produced by the dust of meteors and falling stars, and 
that near positions of certain auroral lines or flutings of manganese, 
lead, barium, thallium, iron, etc., are sufficient to justify us in regarding 
meteoric dust in the atmosphere as the origin of the auroral spectrum. 
Liveing and Dewar have made a conclusive research on this point, by 
availing themselves of the dust of excessive minuteness thrown off 
from the surface of the electrodes of various metals and meteorites by 
a disruptive discharge, and carried forward into the tube of observa- 
tion by a more or less rapid current of air or other gas. These experi- 
ments prove that metallic dust, however fine, suspended in a gas will 
not act like gaseous matter in becoming luminous with its character- 
istic spectrum in an electric discharge similar to that of the aurora. 
Prof. Schuster has suggested that the principal line may be due to some 
very light gas which is present in too small a proportion to be detected 
by chemical analysis or even by the spectroscope in the presence of 
the other gases near the earth, but which, at the height of the auroral 
discharges is in a sufficiently greater relative proportion to give a 
spectrum. Lemstrém, indeed, states that he saw this line in the silent 
discharge of a Holtz machine on a mountain in Lapland. The lines 
may not have been obtained in our laboratories from the atmospheric 
gases on account of the difficulty of re-producing in tubes with sufficient 
nearness the conditions under which the auroral discharges take place. 
Tn the spectra of comets the spectroscope has shown the presence of 
earbon presumably in combination with hydrogen, and also sometimes 
with nitrogen; and in the case of comets approaching very near the sun, 
the lines of sodium, and other lines which have been supposed to belong 
to iron. Though the researches of Prof. H. A. Newton and of Prof. 
Schiaparelli leave no doubt of the close connection of comets with 
corresponding periodic meteor swarms, and therefore of the probable 
identity of cometary matter with that of meteorites, with which the spec- 
