The ‘Forbidden’ Line of Mercury at λ2270 in Absorption


IN a letter to NATURE of May 28, p. 778, I stated that the line λ2270 which is ‘forbidden’ by the selection principle for inner quantum numbers, could not be observed in absorption. I have now repeated the attempt with a more powerful instrument, which has been placed at my disposal by a grant from the Council of the Royal Society. A definite positive result has been obtained, the line showing up clearly in absorption by a column of mercury vapour 45 cm. long boiling at a pressure of 95 cm. It is well seen on several different negatives. The range of conditions for observing it is very limited. Too much vapour blots out the continuous background; too little fails to show the line absorption.

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RAYLEIGH The ‘Forbidden’ Line of Mercury at λ2270 in Absorption. Nature 120, 295 (1927) doi:10.1038/120295a0

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