§ XLIV.— Measurement of Luminosity . In the paper on Colour Photometry (Bakerian Lecture, 1886) a curve of luminosity of the spectrum of the light from the “ crater ” of the positive pole of the electric (arc) light was given. The apparatus used for measuring the luminosity was described in that paper, and certain modifications afterwards made were described in the appendix, and in a further paper with the same title in ‘Phil. Trans.,’ 1888.