On selective absorption by luminous mercury vapour

Abstract
In a previous paper published by the authors there were described, among others, experiments on the reversal of the main component of the 5461 A group of lines and its brightest satellite + 0·085 A in the spectrum of the low-pressure mercury vapour lamp. McLennan, Ainslie and Miss Cale have repeated these experiments and, in the main, confirm the results obtained by us. They, however, point out that what is taken as the main component when the green radiation is examined in instruments of resolving power of the order of 200,000 is a group of five lines of wave-lengths - 0·020 A, - 0·008 A, 0, + 0·008 A, and + 0·018 A; and they state that only the three central lines are absorbed by luminous mercury vapour, the unabsorbed satellites + 0·018 A abd - 0·020 A producing the appearance of reversal. They also describe experiments which, they consider, indicated the absence of absorption in the case of all the other components of this group, including +0·085 A. This last result is at variance with our observation of the reversal of this satellite, recorded in the paper cited above. To clear up this point we undertook a systematic investigation of the absorption of the satellites of the three lines 5461 A, 4358 A, and 4047 A (1 p 1 - 1 s , 1 p 2 - 1 s , and 1 p 3 - 1 s on Fowler's notation) the first triplet of the second subordinate series of triplets. We have observed marked absorption, under suitable conditions, in the case of all the satellites of the group 5461 A, excepting one, viz., -0·237 A, and have reversed five of them. We find that even -0·237 A shows feeble, but distinctly perceptible, absorption under exceptionally favourable conditions. In the present paper we describe the behaviour of the luminous vapour in regared to the members of this group (5461 A, 1 p 1 - 1 s ).

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