By using separated isotopes of mercury 198 and mercury 202, in sealed discharge tubes, the isotope shift in several lines from 16 000 A to 4000 A has been measured with direct recording techniques. A grating crossed with a pressure tuned Fabry-Perot interferometer was coupled to electronic detectors to make a linear recording system. The detectors were a photomultiplier tube in the visible and a lead sulfide cell in the infrared. The output of the detector is amplified and then recorded on a strip chart recorder. By optical switching between the two discharge tubes during the recording both isotopes appear on the same record without overlapping of the intensity pattern. Because of the inherently high accuracy of this type of system the probable errors are small.
FRED McCLUNG and JOHN R. HOLMES, "Isotope Shift in the Spectrum of Mercury," J. Opt. Soc. Am. 47, 297-297 (1957)