The present report may be summarized as follows.
(1) The intensity of ultraviolet rays from an A. C. quartz mercury lamp was investigated with a vacuum photoelectric tube, the sensitive element of which was cadmium, and a tetrode vacuum bulb UX 54. By way of comparison, a brief survey is given on the methods adopted by former investigators in the measurement of ultraviolet light.
(2) The accuracy of the present method was determined by the extent to which the law of inverse proportion of the square of the distance verified.
(3) Some of the effects of a reflector was investigated and found to be great beyond expectation.
(4) For the transient period, just after the lamp was started, the ultraviolet rays were found to behave quite differently in intensity from visible ones. (The latter was measured with a “Mazda Selenium Photo-Cell”.) The intensity-time curve of another quartz mercury lamp (Hanau) showed a curve of an utterly different type.
(5) After the state of equilibrium was reached, and within the range of deviation of the A. C. voltage usually met with in laboratories, the light intensity G may be expressed by the simple equation
G=kVⁿ
V being the effective voltage of the source. The mean value of n for ultrabiolet rays was 3.23, and that for visible ones 3.36.
(6) The foregoing result in regard to the intensity-voltage relation do not agree with the formula proposed by Dr. Asada, who deduced it from his extensive investigation. The author intends to put the equation to test again in the near future, although this type of formula is known to hold good in case of incandescent lamps.

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