This paper discusses the criteria for suitable wavelength standards in the extreme ultraviolet, and presents provisional values (obtained with a two-meter focus normal incidence vacuum spectrograph) for a number of lines of C, N, O and A falling in the spectral range between $\lambda 1850$ and $\lambda 800$. These are compared with the previous results of Edlén and of Bowen and of Bowen and Ingram. Further independent determinations are desirable. The present values, as those obtained by previous investigators, may be subject to a small systematic error inherent in the method of overlapping orders. This source of error is believed to be small, but must be eliminated before permanent standards can be established.

• Received 7 March 1935

DOI:https://doi.org/10.1103/PhysRev.47.653