The upper one is the X-ray emission spectrum of tungsten. It consists of general radiations, corresponding to white light, scattered throughout the whole length of the spectrum as a background and superposed upon these two groups of lines. Fig. 21a—PHOTOGRAPHS OF THE SPECTRA OF THE CHARACTERISTIC X-RAYS FROM CERTAIN SUBSTANCES The remarkable element in these photographs is the exact similarity of the spectra produced by the different elements and the step-by-step shortening of the wave-length (which is proportional to the distance from the line on the left to the spectral lines) as the atomic number increases. This is shown both in the series, which is produced by stimulating the inmost pair of electrons in each atom, and the series, which is produced by stimulating the group of eight electrons in the second ring or shell from the center. Fig. 21b—PHOTOGRAPHS OF THE SPECTRA OF THE CHARACTERISTIC X-RAYS FROM CERTAIN SUBSTANCES [Pg 199] The two lines are here close to the central image, for the wave-lengths are here very short, since tungsten has a high atomic number (74). Farther to the right is the series of tungsten lines which will be recognized because of its similarity to the series in the plate opposite p. 198. Between the and the lines are two absorption edges marked and . The former represents the frequency above which the silver absorbs all the general radiation of tungsten but below which it lets it all through. The latter is the corresponding line for bromine. In a print from a photograph absorption in the plate itself obviously appears as a darkening, transmission as a lightening. Just below is the spectrum obtained by inserting a sheet of molybdenum in the path of the beam, i.e., before the slit of the spectrometer. Absorption in the molybdenum will obviously appear as a lightening, transmission as a darkening. It will be seen that the molybdenum absorbs all the frequencies in the X-ray emission of tungsten higher than a particular frequency and lets through all frequencies lower than this value. This remarkable characteristic of the absorption of X-rays was discovered by Barkla in 1909.[148] The absorption edge at which, with increasing frequency, absorption suddenly begins is very sharply marked.