1908-9.] Negative Attempt to detect Fluorescence Absorption. 401 
XXIII.— A Negative Attempt to detect Fluorescence Absorption. 
By Robert A. Houstoun, M.A., D.Sc., Ph.D., Lecturer on 
Physical Optics in the University of Glasgow. Communicated 
by Professor A. Gray, F.R.S. 
(MS. received March 20, 1909. Read July 12, 1909.) 
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. 
Since that volume was published an article has appeared by 
R. W. Wood * in which he gives an account of a new and ingenious 
method of observing the effect directly — without combining three separate 
* R. W. Wood, “ On a Method of showing Fluorescent Absorption directly if it 
exists,” Phil. Mag., Dec. 1908. 
VOL. XXIX. 
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