1869.] On some Recent Spectroscopic Researches. 211 



ferent conditions of temperature and densitj^ as is well known in the 

 case of nitrogen, the vapour of sulphur, and some other suhstances, 

 for throughout all these changes each gas behaves in a way peculiar 

 to itself. 



As far as our knowledge goes, there appears to be but one 

 exception to the statement that a spectrum of bright lines indicates 

 luminous gas. Bunsen found that when solid erbia is heated to 

 incandescence, the continuous spectrum contains bright bands. 



Third Form. — This type embraces all spectra in which the 

 continuity of the colours of the spectrum are interruj)ted by dark 

 lines or bands. These gaps in the spectrum, which indicate that 

 light of certain refrangibilities is wanting, do not teach us anything 

 of the source of light itself, but show the existence, without the 

 source of light, of vapours at a lower temperature, which, by a 

 selective power of absorption peculiar to them, have quenched the 

 light of certain periods of vibration only, and have not been able to 

 make up for the light they have taken, by light of their own. As 

 the kinds of light absorbed by each vapour correspond precisely 

 with the set of bright lines which that vapour emits when in a 

 luminous state, a comparison of the bright lines of substances that 

 are known with the dark lines seen in a spectrum will show whether 

 the vapours thi'ough which the light has passed are those of any of 

 the bodies with which we are acquainted. The bright spectrum, 

 in which the dark lines occur, must be questioned for information 

 of the source of the light itself, according to the principles stated 

 under the first form of spectra. 



The foregoing statements form the canons of interpretation by 

 which we are to be guided in our explanation of the spectra of the 

 heavenly bodies. 



The most important recent information obtained respecting the 

 fixed stars results from the application of spectrum analysis in a 

 new direction. 



Under certain conditions, which will be stated, the spectrum of 

 a luminous body is adapted to tell us whether that body is moving 

 towards or from the earth. 



It may be well, however, to point out in how remarkable a 

 manner this new application of prismatic analysis supplies a want 

 wliich astronomers had come to regard as one that could not be met 

 by any method of observation within our reach. The stars, though 

 apparently so immovable that they can serve as the figures on the 

 dial-plate of the heavens to which all sensibly moving objects may 

 be referred — the fixed stars, as it is still convenient to call them, 

 are not absolutely motionless, like fiery studs riveted in the canopy 

 of heaven. These brilliant points are found to shift their places to 

 a minute extent relatively to each other. Small displacements are 

 found which must be interpreted to represent a proper motion 



