Fluorescence in violation of Stokes' law, excited in an alcoholic solution of specially purified fluorescein, was compared by a photoelectric method with the reciprocal emission when fluorescent and exciting wave-lengths are interchanged. The following theoretical equation, recently proposed by the author, was found to be largely verified: $\frac{f_{\mathrm{yx}}}{f_{\mathrm{xy}}}=\frac{{\lambda }_x{J}_y}{{\lambda }_y{J}_x}$. Here $f$ is the exciting power, i.e. $f_{\mathrm{yx}}d{\lambda }_y$ is the emission of fluorescent energy from unit volume in the wave-length range $d{\lambda }_y$ when the fluorescing substance is illuminated with unit density of exciting radiation of wave-length ${\lambda }_x$, $f_{\mathrm{xy}}$ is the same thing with the wave-lengths interchanged, and $J_x$, $J_y$ are the densities of black-body radiation for these two wave-lengths at the temperature of the fluorescing substance. A peculiar interest attaches to this equation because it contains no unknown constants.

Fluorescence, excitation and absorption curves for the same fluorescein were also determined with moderate accuracy. The fluorescence curves show a single maximum around 5240A but change shape as the exciting wave-length is varied. The excitation curve for fixed incident energy shows a maximum around 5000 for fluorescence at 5270; for other points in the fluorescence spectrum the maximum excitation occurs at shorter wave-lengths, e.g. at 4730±80A for fluorescence at 5009 or 5582. The curves suggest two superposed bands of different width but with almost coincident peaks. The maximum absorption occurs around 4750.

• Received 15 November 1926

DOI:https://doi.org/10.1103/PhysRev.29.466