This report, the third of a series dealing with the isotopic analysis of uranium, describes the work done with a 22-ft Eagle mounted, direct-reading spectrograph using 5 in. of a 15,000 lines/in. concave grating in the second order. The samples were excited in a multiple source assembly consisting of 4 hollow cathode source lamps and an automatically positioned selector mirror. This arrangement allowed close comparison of unknown and standard, minimized the effect of exit slit misalignment, and allowed large number of routine samples to be handled economically. A complete analysis in duplicate, including comparison standards, was made in less than 20 min. Measurements on 4 standards having U<sup>235</sup> concentrations between 10 and 51% showed average external and internal precisions of ±0.23% and ±0.048% U<sup>235</sup> for a single cathode determination. Analyses of 32 unknown samples containing 10-85% U<sup>235</sup> determined independently on the optical spectrograph and the mass spectrometer, showed good agreement. Comparative measurements on 65 unknown samples in the range of 30-40% U<sup>235</sup> showed an agreement of ±0.17% U<sup>235</sup> with no significant bias. All precisions were expressed at the 95% confidence level. The factor limiting the precision—a weak line interfering with the U<sup>235</sup> measurement, and the effect of U<sup>234</sup> and U<sup>236</sup> on the analysis are discussed.

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