Revisiting the "forgotten" first zoomable refractive x-ray lens

In 1948 Kirkpatrick and Baez showed that two crossed mirrors in a tandem configuration with concave surfaces can focus hard x-rays by reflection. Right afterwards in 1949 Kirkpatrick showed theoretically that x-rays can also be focused by refraction, when they are transmitted through a similar system, i.e. in conditions, when the reflectivity at the concave interface is negligible. He performed an experiment, in which he refracted an x-ray beam at grazing incidence at such a curved interface in one direction. Consequently this was the first reported practical use of a refractive x-ray lens. The experiment was forgotten. Here we compare more systematically the focusing in such devise with the expectations. We propose the use in a flat field spectrometer, which could provide better spectral resolution for fluorescence analysis than the commonly used Si drift diodes, however, on the expense of a rather low efficiency. The application of the system may thus be limited to the spectral characterisation of x-ray sources.