Abstract: A filter apparatus includes a filter element. The filter element is formed of a sheet of gas-porous filter material which has been folded into accordian pleats. Alternate pleat-folds are of tight, near knife-edge definition, and arcuate form definition. Hence, all folds on one side of the accordian-folded material are of the knife-edge form, and all folds on the other side are of the arcuate. The pleated material is turned into a cylindrical conformation, with the arcuate folds confronting the central axis of the cylinder, and the tight, knife-edge folds outwardly directed. An adhesive-backed band circumscribes the cylinder to hold the accordian-folded material in such form. Ends caps, one with a central aperture formed therein, are fixed to the ends of the cylinder. The latter is then positioned and secured under a venturi section or pipe. The venturi section or pipe, in any cross-section thereof, is of rectilinear shape.

Abstract: Transitions from the 2p.sup.4 (.sup.1 S.sub.0)3s .sup.2 S.sub.1/2 state of atomic fluorine to all allowed lower states produces laser emission at six new wavelengths: 680.7 .ANG., 682.6 .ANG., 3592.7 .ANG., 3574.1 .ANG., 6089.2 .ANG., and 6046.8 .ANG.. Coherent radiation at these new wavelengths can be generated in an atomic fluorine laser operated as an amplifier or as an oscillator.