Excitation of laser transitions in He-Kr, He-Ar, Ne-Xe, and He-Ne-Xe mixtures was investigated in hollow-cathode discharges. The dominant excitation mechanism of the noble-gas ions was found to be collisions of the second kind between the ground-state noble-gas ions and helium or neon metastables. Based on gain and absorption spectroscopy measurements, the cross section for this mechanism was estimated to be ⩾10−14 cm2. New laser transitions in noble-gas and copper ions are also reported.

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MS. recezved 1 M a y 1947ABSTRACT. A scheme of measurements is discussed from which absolute determinations of sound field pressure can be made The relation between the scheme and the reciprocity method is discussed. I t is shown that the measurements also yield the axial pressure and " projection efficiency " of a transceiver, the " projection efficiency " being defined as the ratio of the actual axial intensity to the axial intensity as it would be if the transceiver converted the whole energy absorbed into sound by vibrating 2.3 a simple piston. I t is suggested, without proof, that the upper limit of the projection efficiency is unity, and the relation between-this quantity and the efficiency defined as the ratio of radiated acoustic power to consumed electrical power is shown to depend on a directivity coefficient and on the effective area of the radiator face. I t is pointed out that neither of these quantities is readily determinable and that therefore the conversion efficiency of the transceiver, and hence the acoustic power radiated, cannot be obtained.* Security reasons still prevent the publication of actual numerical data, but the principles involved are of general interest in acoustical work and it has been thought worth while to prepare the theoretical work for publication.

We present some experiments and calculations that illustrate the establishment of a cellular Stokes flow between two parallel plates. The structure of this flow is composed of successive closed eddies. The study of the influence of an additional plate mounted midway between the channel walls shows the sensitivity of these eddies to the variation of the geometry. We suggest that this flow might be introduced in the specialized teaching of fluid mechanics, to show examples of transfer by cellular motion.

A highly sensitive technique for measuring high power laser-generated shock-wave velocities is described. Shock-wave velocities produced by pressures as low as 50 GPa were measured, while there is no limit to the pressure that can be measured on the high pressure side.