A series of hollow-cathode tubes has evolved from the original Schüler type, for use at low temperature with valuable small samples (~10 mg). In the final tube assembly the light emerges with a divergence of f:3 through a 35-mm window. The cathode, of Al or Mg (or possibly Be) is clamp-sealed to the tube to be easily removable, yet in direct contact with liquid air. The tube has been used with a static rare gas supply as well as a conventional circulating system. Several quickly interchangeable sources have been used to make relative measurements of isotope shift with an uncertainty of about 2 middot; 10−8.
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Arrøe et al. (1950) studied this question.