The work described in this paper consists of rather careful measurements, but under low dispersion only, of the spectra of twenty gas-vacuum tubes, generally of different gases and illuminated by small induction sparks, but seen very brightly by the end-on method of viewing.

A comparison of the different spectra thus obtained follows, and some curious results are elicited as to the prevalence of certain impurities among gases, as well as alterations and even transformations of some of them with time and use.

These facts are contained chiefly in Appendix 1 and Appendix 2; while a third appendix, kindly contributed by Professor Alexander Herschel, contains some further observations of his with the same apparatus but higher dispersion introduced. See his account of the same.