P. Ohlin has reported irregularities in his continuous x-ray spectrum isochromats taken at low voltage (3000 to 5000 volts) indicating that the continuous x-ray spectrum from thick targets may have, within 10 or 20 volts of the quantum limit, one or more indentations in which actual reversals of slope occur. Unpublished work of Bearden and Schwarz agrees with this. Much less pronounced irregularities about the same distance from the limit were observed at higher voltages (10,000 and 20,000 volts) by DuMond and Bollman much earlier. The present paper interprets these irregularities in terms of the permitted and excluded energy states for the electron in the crystal lattice in its final state after having radiated the x-ray photon. The paper explains (1) how negative slopes in the isochromats from a thick target are possible, (2) why the irregularities are much more pronounced for low than for high voltage isochromats, (3) how the observed isochromat curves may be corrected for the varying retardation of the cathode-ray electrons at varying depths beneath the surface of the target to obtain information regarding the transition probabilities and energy levels for the excluded and permitted empty energy bands for electrons in the target crystal lattice. Some of Ohlin's isochromats are reproduced giving their shapes before and after correction by this method. Confirming evidence from recent work of Davisson and Germer of entirely different nature regarding the position of the first excluded energy band above the Fermi conduction levels is cited.

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