99
because even isotopes have no spectral fine structure. Natural
cadmium is a mixture of even and odd isotopes, but even so
Michelson's choice of the red line was remarkably wise, for the
disturbing effect of the odd isotopes is exceptionally small in this
instance.
An even isotope of mercury is superior to cadmium as a source
but owing to the fact that mercury lamps will not operate below
o° C (2730 K) mercury isotope is inferior to krypton even though
the heavier mercury atoms are slower in thermal motion than
krypton atoms at the same temperature. Figure 1 shows interference
fringes using the green line of the mercury spectrum. The simple
sharp fringe pattern was obtained from a discharge lamp containing
mercury-198 isotope, the complicated pattern was obtained from
an identical lamp filled with natural mercury. The fine structure
due to the odd isotopes present in natural mercury is clearly seen.
Fig. 2. Krypton discharge lamp Cryostat.
