Since the celebrated paper by Kirchhoff, “On the relation between the radiating and absorbing powers of different bodies for light and heat," in which he detailed the remarkable experiments of reversing the lines of lithium and sodium by sunlight and by the vapours of those: metals in the flame of a Bunsen’s burner, and mentioned the reversal of the brighter lines of potassium, calcium, strontium and barium when the deflagration of the chlorates with milk-sugar was used instead of the flame of a Bunsen’s burner, further researches in the same direction have been made by Cornu, Lockyer, and Roberts. The method adopted by Cornu, which had been previously used by Foucault, is one of great ingenuity, dependent upon so arranging the electric arc that the continuous spectrum of the intensely heated poles is examined through an atmosphere of the metallic vapours volatilized around them. By this means Cornu succeeded in reversing several lines in the spectra of the following metals in addition to those above-mentioned, viz., thallium, lead, silver, aluminum, magnesium, cadmium, zinc, and copper. He observed that in general the reversal began with the least refrangible of a group of lines, and gradually extended to the more refrangible lines of the group; and he drew the conclusion that a very thin layer of vapour was sufficient for the reversal. It may be noted that in almost every case the lines seen by him to be reversed were the more highly refrangible of the lines characteristic of each metal, confirming generally the opinion expressed by Stokes in a letter to Lockyer in the Proceedings of the Royal Society for 1876, in which he introduces for the first time the idea of the persistency of different rays with reference to temperature. The method adopted by Lockyer in the first instance was to view the electric arc through the vapours of the metals volatilized in a stream of hydrogen in a horizontal iron tube.