About two years ago, in the spring of 1895, in the course of a conversation with Professor J. H. Poynting on the nature of the phenomenon of fluorescence, in the study of which I was at the time beginning to engage, the suggestion was thrown out by him that possibly fluorescent bodies absorb differently, according as they are fluorescing or not, the rays which they give out whilst fluorescing, thus that a body A would absorb differently, according as it is fluorescing or not, the rays from a similar body B in a state of fluorescence. Some fluorescent bodies undoubtedly do, others do not, absorb, except to a very small extent, the rays which they emit. A strong solution of fluorescin or eosin, for instance, hardly permits its fluorescent light to penetrate even a very small thickness. Glass coloured with oxide of uranium is much more transparent, but sulphate of quinine hardly absorbs these rays at all. The question was whether during the act of fluorescing any change is produced in the nature of the absorption itself, that is, whether during fluorescence there is an increase or diminution of absorption in that part of the spectrum where the emitted rays lie. For instance, with uranium glass the radiation takes place chiefly between the D and E lines, so that the absorption power for rays may be different according as the body is examined in the dark or in daylight in this part of the spectrum. Of the five bright bands of which the radiation consists, three lie between the D and E lines, the other two being of less refrangibility and of less intensity in the red and orange (Stokes, ‘Phil. Trans.,' 1852). With the spectroscope I have used I have not been able to see the band in the red, but the other four were quite distinct. The spectrum seemed to me to be of the nature of maxima and minima.