. The extreme confusion which the
spectra of the lines of the gases seemed to present is now, in great
part at least, cleared up. Balmer gave some time since, in the case of
the hydrogen spectrum, an empirical formula which enabled the rays
discovered later by an eminent astronomer, M. Deslandres, to be
represented; but since then, both in the cases of line and band
spectra, the labours of Professor Rydberg, of M. Deslandres, of
Professors Kayzer and Runge, and of M. Thiele, have enabled us to
comprehend, in their smallest details, the laws of the distribution of
lines and bands.

[Footnote 47: Many theories as to the cause of the lines and bands of
the spectrum have been put forward since this was written, among which
that of Professor Stark (for which see _Physikalische Zeitschrift_ for
1906, passim) is perhaps the most advanced. That of M. Jean Becquerel,
which would attribute it to the vibration within the atom of both
negative and positive electrons, also deserves notice. A popular
account of this is given in the _Athenaeum_ of 20th April 1907.--ED.]

These laws are simple, but somewhat singular. The radiations emitted
by a gas cannot be compared to the notes to which a sonorous body
gives birth, nor even to the most complicated vibrations of any
elastic body. The number of vibrations of the different rays are not
the successive multiples of one and the same number, and it is not a
question of a fundamental radiation and its harmonics, while--and this
is an essential difference--the number of vibrations of the radiation
tend towards a limit when the period diminishes infinitely instead of
constantly increasing, as would be the case with the vibrations of
sound.

Thus the assimilation of the luminous to the elastic vibration is not
correct. Once again we find that the ether does not behave like matter
which obeys the ordinary laws of mechanics, and every theory must take
full account of these curious peculiarities which experiment reveals.

Another difference, likewise very important, between the luminous and
the sonorous vibrations, which also points out how little analogous
can be the constitutions of the media which transmit the vibrations,
appears in the phenomena of dispersion. The speed of propagation,
which, as we have seen when discussing the measurement of the velocity
of sound, depends very little on the musical note, is not at all the
same in the case of the various radiations which can be propagated