Memoirs presented to the Cambridge philosophical society on the occasion of the jubilee of Sir George Gabriel Stokes, bart., Hon. LL. D., Hon. SC. D., Lucasian professor.

OF SELECTIVE ABSORPTION OF LIGHT BY A GAS. 363
WTe nray inquire what value of the dissipation-ratio I would make the intensity diminish
in the ratio 1/e in the distance of a wave-length. If we write
I =.3 X2....................................... (102),
so that I,, denotes the maximum value of the dissipation-ratio for n = 1, the requisite
value is given by
I 2
I_.7r_....................................... (103).
im 3
On our previous numerical assumptions this is about 4 x 10-7. The corresponding value
of e in (95) is about 4 x 10-7. This is comparable with, although distinctly less than,
the virtual variation of wave-length which takes place, on Doppler's principle, in a gas
with moving molecules, and which is held to be sufficient to explain the actual breadths
of the Fraunhofer lines. Having regard to the very much sharper definition which we
ieet with in the vibrations of the first type, and to the increase of sharpness (in each
type) with the index n of the mode considered, it would appear that there is no
primcâ facie difficulty in accounting, on our present hypothesis, for absorption-lines of such
breadths as occur in the actual spectrum.
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