1964AJ69Q . 534B 534 ABSTRACTS extragalactic or probably extragalactic. A complete catalogue of sources will be published in the near future. Between 50 and 60 additional sources are available using the NRAO 300-ft instrument and it is expected measurements will be made on these additional sources during the latter half of 1964.

The unique optical system of the folding single-lens-reflex viewfinder used in the Polaroid SX-70 Land camera has required novel approaches to design and manufacture. The camera uses an unusual short-barrel taking lens with front-element focus, two plane mirrors, an eccentric reflective Fresnel focus screen, an aspheric aperture element, an aspheric concave mirror, and an aspheric eye lens. All the viewing components are tilted or decentered, and two aspheres are not figures of revolution. Beginning with J. G. Baker's computer design, special technology has been needed for producing millions of replicas of this system and controlling their quality.

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Several examples of acoustic resonance in the frequency range from 0.001 to 1.0 Hz have been observed in limestone caverns. In some cases, the cavern geometry is simple enough for direct application of the Helmholtz resonator theory, and good agreement is found. Three experiments are described. Spectral analysis indicates that subacoustic resonance is responsible for numerous reports of periodically fluctuating or reversing cavern winds, and that appropriate wind measurements can provide new information about chambers not accessible to exploration.

The balloon‐telescope flight of February 21, 1964, using reflected radiations near 1.1 μ, showed that the upper Venus atmosphere contains water vapor in amounts comparable to those in similar levels of the earth's atmosphere [Strong, 1961; Bottema et al., 1964a, 1965]. Solar radiation reflected by the Venus clouds in the infrared spectrum region 1.7 to 3.4 μ [Bottema et al., 1964b] was measured on a subsequent flight (on October 28, 1964). This Venus spectrum (curve 1) and the reflection spectrum of a laboratory ice cloud (curve 3) are shown in Figure 1. From the similarity we conclude that Venus clouds are composed of ice crystals.