Note 195 — Chapter V · PROPERTIES OF THE RADIATIONS.
Reading edition
In a recent paper, Giesel (Ber. d. D. Chem. Ges. No. 3, p. 775, 1905) has shown that the bright lines are due to didymium, which is present as an impurity. Exposure of didymium to the radium rays also causes the appearance of the lines.