Short-wavelength light presents special difficulties for researchers. Hard x-rays have wavelengths thousands of times smaller than visible light, enabling them to skip the boundaries of common optical substances without changing their path. To x-rays, all materials have refractive indices close to 1, so the construction of traditional lenses, mirrors and filters to control them is all but impossible. A team at the European Synchrotron Radiation Facility, however, has devised a technique that circumvents these problems. Reporting in the March 23 issue of Nature, the researchers describe how Bragg scattering of x-rays from a silicon crystal lattice can be used to construct a resonator for short pulses at these short wavelengths. The development may lead to the production of an x-ray Fabry-Perot etalon. The group -- which includes researchers from the Institute of Crystallography and Structural Physics at the University of Erlangen-Nuremberg and Motoren und Turbinen Union GmbH in Munich, Germany -- machined a 150-mm cavity into a monolithic silicon crystal. Because the interatomic spacing of the atoms in the crystal lattice is on the order of 0.5 nm, it could store even the 100-ps, 0.78-Å x-rays produced by a synchrotron for up to 14 cycles. No effective medium for amplifying light at these wavelengths is known, so it is unlikely that the resonator could soon be used to build an x-ray laser. By changing the spacing of the silicon walls in the cavity, however, the researchers believe that they will be able to select specific wavelengths from the relatively broad spectrum produced by the synchrotron. Coupling the device to the free-electron lasers that have been proposed in this range could further narrow the linewidth, enabling greater precision in x-ray crystallography and x-ray spectroscopy.