Spectroscopists have observed the spectrum of the element mercury with interest. Because monochromatic light from one of the stable mercury isotopes (mercury 198) was considered as an international primary standard of wavelength for comparison with the standard meter, considerable experimental work has been devoted to this one isotope. However, the other six stable isotopes (mercury 196, 199, 200, 201, 202, and 204) received a fair amount of attention too, since measurements and observations of spectral lines of all mercury atoms are necessary to better understand physical and chemical properties of mercury. A convenient way for workers to obtain the emission spectrum of mercury is to excite in it enclosed tubes with or without electrodes.
Joseph A. Goleb, "The Use of a Permanent Mercury Source for Spectroscopic Studies of Varying Amounts of Mercury," Appl. Spectrosc. 25, 94-96 (1971)