ABSTRACT
The 1996 centenary of W. K. Rontgen's discovery of X-rays celebrated the penetrating visions offered by these "strange rays." X-ray diffraction, first reported in 1912, connects X-ray wavelengths with the dimensions of crystal lattices, but fails to connect either of these scales with the dimensions of macroscopic objects. The first combined X-ray and optical interferometer was realized at the National Bureau of Standards (NBS). The first generation of γ-ray measurements at NBS used radioactive sources activated in the NBS reactor. Responding to a generally accepted need to improve this reference transition, NBS mounted an effort to extend the optically based measurement chain into the γ-ray region. The crystal spacing was determined with respect to the optical reference in the X-Ray and Optical Interferometry (XROI) measurement and then used, through absolute angle measurement, to link the optical "standard" to X-ray transition wavelengths.
