(a) Microphotometric recording of two photographic films obtained using the (100) reflection of quartz in the 8 Å region of frequency (ν): the upper one, marked (a), was recorded on a 40 µm-thick beryllium plate set in the absorber position of the spectrometer and shows that the synchrotron radiation was too weak to pass through such a thin plate of a light atom; the lower one, marked (b), was recorded on a 7 µm-thick Al foil at the same position. In this second image one sees two regions indicated as I and II, separated by the very strong absorption discontinuity (actually a strong change in the film darkening), which corresponds to the Al K-edge at 7936 Å in frequency (or 1562 eV in energy). Now the abscissa scale would be the energy (E), and the edge value 1559.6 eV. The ∼2.5 eV energy difference is a measure of the accuracy attained by these pioneering measurements (Williams, 2001; the current reference value is 1559.6 eV). The spectra have been recorded during a sequence of exposures during 80 s; they are manifestly overexposed, with the exception of region II. The picture is fuzzy in the original (Cauchois et al., 1963a, p. 411, Fig. 2, left) and has not been retouched. (b) Microphotometric recording as a function of frequency ν of the blackening of a film with the LIII main absorption edge of copper at 13.3 Å (corresponding to 932 eV in energy E, to be compared with the current value of 932.7 eV; Williams, 2001). The precision of the measurement is much better, although no details on the thickness of the foil nor on exposure time are given. The original picture is full of fine spots all over (Cauchois et al., 1963a, p. 411, Fig. 3), possibly due to dust present in the developing or in the fixing bath, and has not been retouched. The edge appears as a fairly large dip over a downward-sloping absorption line, as typically happens when converting plate blackening into graphs. The smaller dips nearby may be related to trace impurities of other atoms.