An x‐ray spectrometer was designed with the first crystal mounted on an arm supported by the frame of the spectrometer, and with the second crystal mounted on the central table of the instrument, whose position is read from a precision circle. The ionization chamber (of 28 cc capacity, filled with krypton) is on an arm whose position is read by a second precision circle. The instrument was built by the Societe Genevoise de Physique.The first crystal was adjusted to throw the K&agr;1line of molybdenum over the main axis of the spectrometer, and measurements were made with the second crystal in the (1, −1), (1, +1), (1, −4) and (1, +4) positions (Allison's notation). The reflection maxima from calcite (corrected to 18° C) occur at &thgr;1=6°42′35.″5 and &thgr;4=27°51′33.″0 with a probable error of 0.″25 due chiefly to errors in reading the circle. Using an apparent grating space for the first order of 3.02904A at 18° C, we get &lgr;=707.830±.002 mA. Larsson obtained 707.831±.003 mA using Siegbahn's photographic method. Comparison of &thgr;1and &thgr;4gives for the index of refraction in calcite, 1 −&mgr;=(2.10±.15)×10−6, as compared with Hatley's value of (2.04±.09)×10−6.