Very weak optical signal measurements using photon counting method with two avalanche photodiodes (APD) cooled at the liquid nitrogen temperature have been made. Variation of the photoelectron pulse countrates and the signal-to-noise ratio S/N for the photoelectron pulses with the optical input intensity have been examined, Analytical considerations are given on the minimum detectable photoelectron Pulse count rates and signal-to-noise ratio, based on the multiplicaton mechanism and the statistical nature of the APD. The optimum operation of the APD taking into account the photoelectron pulse height distribution allows us to the linear detection of the optical input intensity from approsimately 10^-10W to 10^-16W as a function of the photoelectrom countrates.

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To investigate the mechanism of the saccadic stability of the visual system, the probability of perceiving the motion of the test stimulus was measured when the test and inducing stimuli shifted independently during a saccadic eye-movement. When the probability was plotted against the shift size of the inducing stimulus for a constant shift size of the test stimulus as parameter, the probability curves showed the dependence on the shift size of the inducing stimulus and took a minimum value where the test stimulus and the inducing stimulus had the same shift size and direction. Further, the curves were raised when the shift size of the test stimulus was increased. Those results indicate subjects perceived the test stimulus motion not only for the absolute shift of the test stimulus but also for the relative shift between the two. On the assumption that the probability of perceiving the test stimulus motion was a function of only the sum of the absolute shift of the test stimulus and the relative shift between both stjmuli, the theoretical probability curves were obtained, which explained the present results nicely. It was concluded that both the extra-retinal signal and the retinal signal were utilized for the process underlying the saccadic stability.

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This paper reports a method of remote optical measurement of the amount of H₂O emission from volcanoes using Mie-scattering scheme laser radars or simple photographic techniques. From the diffusion pattern and the velocity of the plume containing water droplets, the H₂O massemission rate is derived assuming a saturated humidity condition in the evaporated region in the plume. The photographic technique was first applied to demonstrate the feasibility and the accuracy of this novel method by monitoring H₂O plumes from a stack of the electric power plant. The H₂O mass-emission rate obtained by this remote technique was found to agree well with the direct sampling data. Furthermore, this method was applied to measure H₂O plumes of the volcano Usu in Hokkaido, Japan. From these results it is confirmed experimentally that this method offers a simple and useful technique for remote measurement of the H₂O emission rate from volcanoes.

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