MolybdenumL-Series Wave-Lengths by Ruled Gratings

With plane ruled gratings having 30,000 lines per inch and 14,400 lines per inch and grating to photographic plate distances up to a meter in length in vacuo, about 160 plates were taken of the Mo $L$-series lines. The wave-lengths for $L{\alpha }_1$, and $L{\beta }_1$, as given by Siegbahn using a gypsum crystal are 5.3943A and 5.1658A. If a calcite crystal were employed and approximate correction made for refraction, wave-lengths 5.3960A and 5.1674A should be expected, whereas the average of the ruled grating measurements gave 5.4116A and 5.1832A. A comparison of the two values leads to an apparent value of the electronic charge 4.8162×${10}^{-10\mathrm{}}$ e.s.u., being slightly higher than Bearden's value using Cu $K$ lines. The possibility of the variation being due to anomalous variation in the refractive index of the crystal is discussed.