where _n″_ and _n′_ can assume integral values. This was done since it was not to be believed that the spectral properties of chemically different elements could be so similar. This view was very much strengthened when Fowler, in 1912, discovered the Pickering lines in the light from a vacuum tube containing a mixture of hydrogen and helium. It could not quite be understood, however, why the new lines did not in general appear in the hydrogen spectrum. According to the Bohr theory for the hydrogen spectrum it was impossible—except by giving up the agreement (cf. p. 129) with electrodynamics in the region of high orbit numbers—to attribute to the hydrogen atom the emission of lines corresponding to a formula where the whole numbers were halved. The formula given above might, however, also be written as ( 1 1 ) ν = 4K(------ - ------). ( _n″_² _n′_² ) If the earlier calculations had been carried out a little more generally, _i.e._, if instead of equating the nuclear charge with 1 elementary electric quantum _e_, as in hydrogen, it had been equated with N_e_ where N is an integer, then the frequency might have been written as ( 1 1 ) ν = N²K(------ - ------). ( _n″_² _n′_² ) This formula is evidently the same as that just given when N equals 2. Now we know that helium has the atomic number and nuclear charge 2 (cf. p. 90); a normal neutral helium atom has two electrons and it is, therefore, very different from a hydrogen atom. If, however, a helium atom has lost one electron and therefore has become a positive ion with one charge, it is a system like the hydrogen atom with only one single electron moving about the nucleus. It differs in its “outer” characteristics from the hydrogen atom only in having a nuclear charge twice as great, _i.e._ its spectral formula must be given with N = 2, or N² = 4. The formula for the supposed hydrogen lines would consequently fit the case of a helium atom which has lost an electron. Bohr was aware of this, and he therefore suggested that the lines in question were due, not to hydrogen, but to helium. At first all the authorities in the field of spectroscopy were against this view; but most of the doubt was dispelled when Evans showed that the lines could be produced in a vacuum tube where there was only helium with not a trace of hydrogen.