In our first paper on this subject we have shown how the luminosity of the spectra of various sources of light can be measured; and the present paper is an extension of the subject, dealing with the measurement of the light reflected from bodies in terms of the colours of the spectrum of the light illuminating them. By the method which we adopted in the first part of "Colour Photometry” this can be effected, and, indeed, we carried that out in several instances. The method then employed was very simple. If we wished to measure the illuminating value of the spectrum of light reflected from a metal, we placed it at an angle in front of the slit of the spectroscope, so as to reflect the light from the crater of the positive pole of the electric light through the photometer, and measured the luminosity of each part of the spectrum thus formed by the method we indicated in our paper. Again, in experimenting with Gorham’s discs, such as Maxwell employed, where it became necessary to determine the light reflected from the different coloured papers or cards used in the discs, the plan first adopted was to replace the receiving shadow screen of zinc oxide (see § VI) by the coloured papers, and again to make a luminosity measurement. This plan answered its purpose, but it was rather laborious. When two or three colours are combined by rotation to form a grey, and black and white sectors are combined to match that grey, in order to ascertain the total luminosity of each colour, the angular value of the sectors being known, it is necessary to refer the luminosity to that of some standard reflecting surface, which is naturally a white one. As the comparison light is coloured by falling on coloured paper, the value of the spectrum reflected from such paper could not by this first method be directly compared with that reflected from the white screen. In the case of a coloured screen, the curve of spectrum luminosity would therefore have to be reduced to that in which the comparison light was white. This difficulty was surmounted by making half the receiving screen white and half of the colour whose luminosity was to be measured, illuminating the shadow of the rod thrown on the coloured paper by the spectrum colour, and that thrown on the white card by the white light reflected from the surface of the first prism (§ XXVI). This did away with any reduction or calculation; but still an objection remained, as, for definite comparison, it was almost necessary that the same observer should always make the measurement.