( CELESTIAL SPECTROSCOPY. 
Spectroscopic conditions.—It is now some thirty years since the spec- 
troscope gave us for the first time certain knowledge of the nature of 
the heavenly bodies, and revealed the fundamental fact that terrestrial 
matter is not peculiar to the solar system, but is common to all the 
stars which are visible to us. 
In the case of a star such as Capella, which has a spectrum almost 
identical with that of the sun, we feel justified in concluding that the 
matter of which it is built up is similar, and that its temperature is 
also high, and not very different from the solar temperature. The task 
of analyzing the stars and nebulie becomes however one of very great 
difficulty when we have to do with spectra differing from the solar type. 
We are thrown back upon the laboratory for the information necessary 
to enable us to interpret the indications of the spectroscope as to the 
chemical nature, the density and pressure, and the temperature of the 
celestial masses. 
What the spectroscope immediately reveals to us are the waves 
which were set up in the ether filling all inter-stellar space, years or 
hundreds of years ago, by the motions of the molecules of the celestial 
substances. As a rule,it is only when a body is gaseous and sufficiently 
hot that the motions within its molecules can produce bright lines and 
a corresponding absorption. The spectra of the heavenly bodies are 
indeed, to a great extent absorption spectra, but we have usually to 
study them through the corresponding emission spectra of bodies 
brought into the gaseous form and rendered luminous by means of 
flames or of electric discharges. In both cases, unfortunately, as has 
peen shown recently by Profs. Liveing and Dewar, Wiillner, E. Wiede- 
mann and others, there appears to be no certain direct relation be- 
tween the luminous radiation as shown in the spectroscope and the 
temperature of the flame, or of the gaseous contents of the vacuum 
tube—that is, in the usual sense of the term as applied to the mean 
motion of all the molecules. In both cases, the vibratory motions with- 
in the molecules to which their luminosity is due are almost always 
much greater than would be produced by encounters of molecules hav- 
ing motions of translation no greater than the average motions which 
characterize the temperature of the gases as a whole. The tempera- 
ture of a vacuum tube through which an electric discharge is taking 
place may be low, as shown by a thermometer, quite apart from the 
consideration of the extreme smallness of the mass of gas, but the 
vibrations of the luminous molecules must be violent in whatever way 
we suppose them to be set up by the discharge; if we take Schuster’s 
view that comparatively few molecules are carrying the discharge, and 
that it is to the fierce encounters of these alone that the luminosity is 
due, then if all the molecules had similar motions, the temperature of 
the gas would be very high. 
Soin flames where chemical changes are in progress, the vibratory 
motions of the molecules which are luminous may be, in connection with 
