TUNABLE X-RAY WAVEGUIDE WITH AN AIR GAP. At synchrotron light sources, electron beams make floods of x rays which must be tamed before they can be used in experiments where typically they probe the structure of some tiny biological sample. One of the ways to focus the beam onto the sample is to compress it and guide the x rays through a thin strip of material sandwiched between reflecting surfaces. Usually the guiding material, often carbon, absorbs a substantial portion of the x rays. Researchers at the University of Amsterdam (Friso van der Veen, vdveen@wins.uva.nl, 011-31-20-525-6330) have now produced a waveguide out of two parallel reflecting plates with only air in between. This not only greatly reduces x ray losses but also, when the gap is filled with liquid, permits the x-ray study of lubricants and colloids. In optics geometry is destiny; the coherent wave pattern in the Amsterdam device can be tuned by prising apart the two flat plates which form the body of the waveguide. For the whole process to work the plates (only about 250 nm apart) must be extremely parallel, the equivalent of suspending one soccer field over another at a height of about 5 mm. (M.J. Zwanenburg et al., Physical Review Letters, 22 Feb; see figure at Physics News Graphics.)