of Metallic Spectra in the Ultra- Violet. 343 



for 1885 and 1887, is an extremely valuable contribution to 

 the subject of spectrum analysis ; it contains the material for 

 future generalization in regard to the molecular structure of 

 so-called elements, or in regard to the harmonic relations 

 which may exist between their wave-lengths. Here can be 

 found in juxtaposition the results of various observers upon 

 the metallic spectra of the same metal, and the student can 

 judge of the relative accuracy of the results. A superficial 

 inspection of this Catalogue will show that even distinguished 

 observers, like Thalen and Kirchhoff, often differ in their 

 results by one part in 4000, or one part in 2500. No observer 

 of metallic spectra gives results to more than one tenth of 

 Angstrom's unit, or to more than one tenth of one wave-length. 

 Physical science, however, now demands a greater degree of 

 accuracy. Various hypotheses in regard to the apparent 

 coincidences between lines of metallic spectra and lines in the 

 solar spectrum have been propounded, and can only be settled 

 by more accurate measurements of wave-lengths. There are 

 also questions constantly arising in regard to the displacement 

 of lines of spectra due to the motion of the stars and to 

 changes of temperature, which require a greater degree of 

 accuracy in the measurement of wave-lengths of gaseous and 

 metallic spectra than the results of previous observers afford. 

 It may be remarked, that observations upon the metallic 

 spectra of metals from the limits of the visible red to the 

 limits of the visible violet have become comparatively easy ; 

 for the solar spectrum can be used to identify the lines of the 

 metals, and to ascertain the wave-lengths. It is only in the 

 extreme infra-red region and in the ultra-violet that such ob- 

 servations become difficult. In these regions we must trust to 

 photography to reproduce, by long exposures of the sensitive 

 plate, the feeble lines of metals which may manifest themselves 

 there. In the infra-red region, as far as wave-length 10,000, 

 it is possible to photograph the solar lines, and we can com- 

 pare the spectra of such metallic lines as may exist between 

 the A line and the limit 10,000 with the solar spectrum. 

 Beyond this limit, and beyond wave-length 2800 in the violet, 

 the solar spectrum disappears, and the problem of measuring 

 the wave-length of metallic lines which extend beyond these 

 limits becomes a difficult one. 



Besides the resolution of the difficulty of measuring the 

 wave-lengths of the invisible rays of light with proper accu- 

 racy, the measurement of such wave-lengths is destined to 

 prove a crucial test for various theories which must arise in 

 the progress of physical science. The lines of the metals are 

 exceedingly numerous in the ultra-violet region, far more so 



