Electromagnetically enhanced Hg<sup>202</sup>, as a source material for highly monochromatic radiation, is discussed briefly with relation to the effects of the isotopic impurities. A Fabry-Perot interferogram showing line purity in the 98.06 percent Hg<sup>202</sup> is reproduced.
J. RAND MCNALLY, JR., PAUL M. GRIFFIN, and L. ELKIN BURKHART, "The Possibility of Enhanced Hg202 as a Source of Monochromatic Light," J. Opt. Soc. Am. 39, 1036-1037 (1949)