I have been lately requested to determine the distribution of energy in the spectrum of an argand burner, and have been able to do this by means of the apparatus and methods previously employed at the Allegheny observatory for mapping the invisible spectrum of the sun. The results are curious; and, in the hope that they may also be found useful, I desire to communicate them to the academy. The difficulty is such a determination lies in the mapping of something which is wholly invisible; and it has not been made before, I presume, in spite of its economical importance, because there has been no means known of measuring this invisible energy, except in a rough way, by the thermometer or themopile, by a process which gives incomplete results.

It was my object not merely to indicate how much of the radiation from a gas-burner was visible, and how much was not, but to give a map of its distribution on the normal or wave-length scale, which would enable any one to see the quality and amount of the energy in each part of the light and head region.

The ordinary argand burner, burning common house-gas within a glass chimney, was first placed at the centre of the curvature of a large Rowland concave grating; and, but means of the bolometer, the head was measured at successive points in the spectrum down to a wave-length of about.001 mm., where the overlapping second spectrum began to be sensible.