For the purpose to measure the decay of the persistent phosphorescence of x-ray fluorescent screens, a special single sweep oscillograph unit-set was constructed. In order to desribe the input signals caused respectively by both the excited brilliant lumines cence and the feable after-glow of x-ray screen on a same Braun-tube screen, the replacement mechanism of neutral optical filter to be inserted between a photomultiplier and x-ray screen, is interlocked with the motion of switches of a electric circuit for x-ray ernittion, a photomultiplier output circuit, and a single sweep operation circuit.
Oscillograms of after-glow decay of x-ray fluoroscopic screens and radiographic intensif ying screenswere recorded by the above mentioned single sweep oscillograph, and the ratios of after-glow intensity to that of luminescence intensity of screens excited by x-rays were measured. The relations between visible after-glow intensity of x-ray screens and after-image of human eye weve studied, and intensity of after-glow of several kinds of commercial intensif ying screens was mutually compared.
Oscillograms for the various fluorescent materials were studied, and characteristic decay curves of persistent phosphorescence were plotted on log-log or semi-log scales. Oscillograms of CaWO₄ (radiographic intensifying screen) and CaWO₄·Pb indicate simple exponential response, and persistent phosphorescence of ZnS·Ag (photographic and radiographic), ZnCdS·Ag (fluoroscopic) and BaSO₄·Pb (radiographic intensifying screen) were found to obey power law factor decay.

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On colored light sources threshold illuminance was measured by laboratory and field experiments. In the laboratory a light source of the visual angle 50 seconds of are was observed at a distance of 7 ni against neutral background. Background luminances were 10.2, 2.0 and 0.1 cd/m², which correspond to the luminance in the moonlight. By field experiments a Hight source of the visual angle less than 50 seconds of arc was observed at 500m. The background was natural objects. Its luminance and chromaticity were measured by a telephotometer. Background luminances were 2, 300 and 320 cd/m².
For the light source three red filters, four yellow-red filters, one yellow filter and five green filters of various dominant wavelengths and purities were applied.
The experimental result shows that the threshold illuminance, ε_m for perceiving the brightness of the light source, has nearly the same value for different colors and depends on the background luminance. Author's data coincide approximately with the Tiffany Foundation's, Green's, and Knoll, Tousey and Hulbert's.
The threshold illuminance, ε_m, to perceive the chromaticity, increases when the chromaticity difference between source and background decreases, especially when the background luminance is high. It depends also on the hue of the source, and for the same chromaticity difference ε_me for yellow and yellow-red is higher than that for green. Representing the chromaticity difference by the distance d between the points for the chromaticities of the soure and the background on Breckenridge and Schaub's RUGS coordinates, an empirical formula d (ε_m) ^1/2=k is obtained; k increases with background luminance and is greater for yellow and yellow-red than for green.

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