Optical behavior of the atmosphere in long path interferometry is investigated experimentally employing a specially designed He-Ne laser interferometer with optical path length of a few ten meters. Experiments are made on a rock in a tunnel of the Matsushiro Seismological Observatory. The variation of the interference order of the interferometer is measured and recorded on a chart in terms of the mean refractive index of the atmosphere together with the atmospheric pressure and temperature which are measured at the midst of the optical path of the interferometer. Experiments show that in such a long path interference the change of the mean refractive index is proportional to the change of the atmospheric pressure, but not to the atmospheric temperature measured at the midst of the path. When the temperature is stable, the change of the mean refractive index can be corrected with a variation less than 2×10^-8.

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In order to form a three-dimensional image from a set of two-dimensional images, a simple method using a fly's-eye lens is proposed. The depth perception obtained from the reconstructed image using the fly's-eye lens is analysed, and an optical configuration which maximizes it is proposed. According to this configuration the threedimensional image composed of the set of cross-sectional images is reconstructed.

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A double half-wave filter, whose center wavelength is 600nm and consists of nineteen layers of zinc sulphide and magnesium fluoride, was fabricated with an improved crystal film thickness monitor. With a little corrected monitored values of the change in oscillation period of the crystal, a satisfactory double-peaked spectral characteristic of the filter was obtained. The separation of the two transmission peaks was-16.5nm. The peak transmittance was 79% including the reflection loss on the rear surface of the filter.

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