In an attempt to fill up the gap between the shortest ultra-violet light waves hitherto produced and the longest X-ray waves known, Hughes recently made a study of the characteristic X-rays emitted by carbon and by boron when bombarded by electrons. In this investigation the energy of the bombarding electrons was increased by steps, and the critical values were determined that were necessary and just sufficient to cause the bombarded element to emit its characteristic radiations with measurable intensities. These characteristic radiations were detected, and their intensities measured, by their photo-electric action on an insulated electrode of nickel or of silver. The method followed by Hughes in recording his results was to plot curves with the values of the accelerating potentials of the electrons as abscissæ and the measures of the photo-electric effect divided by the corresponding electronic currents as ordinates. At certain critical accelerating voltages it was found that these curves showed marked and abrupt kinks or changes of curvature, and these changes were taken to connote the beginning of the emission by the bombarded element of its characteristic radiations.
On the excitation of characteristic X-rays from light elements
