According to Kirchhoffs law every body in a state of pure temperature radiation absorbs those rays which it emits, and the ratio of the coefficient of absorption to the coefficient of emission is constant for all bodies. In many cases which do not come under the category of temperature radiation —for example, a sodium flame—the wave-lengths emitted are also absorbed. It is thus possible that a fluorescing substance, in addition to its ordinary absorption, may absorb while fluorescing those wave - lengths that it fluoresces. From a photometric study of fluorescent uranium glass Burke stated he had discovered such an effect. A cube the edges of which measured 1 cm., transmitted while fluorescing only 57 per cent, of the light it transmitted when not fluorescing, the measurements being made, of course, on light of the wave-length emitted during fluorescence. Nichols and Merritt investigated the subject in detail, obtained a similar result for fluorescein, using a Lummer-Brodhun spectrophotometer, and found that the additional absorption produced during fluorescence—or, to give it its usual name, fluorescence absorption—possessed many strange properties; for example, it did not follow the exponential law. Camichel repeated Burke's work with two different experimental arrangements, and found no effect; he also obtained no effect with fluorescein. Miss Wick, using the same apparatus and working in the same laboratory as Nichols and Merritt, confirmed their results for resorufin. For a full description of the work of these observers and an account of the present state of the question, together with the necessary references, the fourth volume of Kayser's Spectroscopie (1908), pp. 963–973, should be consulted.