THE SIGNIFICANCE OF SPECTROSCOPY 233 



The remarkable thing is that, with the hydrogen atom 

 containing only one electron, the radiation calculated from these 

 speculations agrees absolutely with the observed spectrum, 

 and gives Balmer's formula completely, with variations not 

 exceeding the probable errors of observation. Moreover, 

 the supposed atom has made more than one remarkable 

 prediction, which experiment has verified up to the hilt. 

 We can understand now how it comes about that a wave- 

 number appears as a difference of two terms. According to 

 the theory, each term is proportional to the energy of an 

 electron in an orbit, so that the difference of two terms, like 

 the wave-number, is proportional to the difference of the 

 corresponding quantities of energy. It is unfortunate that 

 the mathematical difficulties connected with the problem of 

 three bodies prohibit us from investigating the possible radiation 

 from an atom containing more than one electron and a nucleus. 

 We are, therefore, almost entirely restricted to hydrogen for 

 detailed tests of the theory. Almost so restricted, but not 

 quite. The helium atom has two electrons, but if, by any 

 means, we can abstract one of them, the remaining system 

 is amenable to investigation, and is, in fact, just like the atom 

 of hydrogen, except for the double charge and correspondingly 

 greater mass of the nucleus. The radiation to be anticipated 

 from such a system has actually been obtained by passing 

 very strong electric discharges through a tube of rarefied helium, 

 and the complete agreement of prediction and observation, 

 even to the small modification of the series constant for helium, 

 is one of the reasons why it is felt that, despite its apparent 

 incredibility, the theory must lie very close to the truth. It 

 appears that the spectrum of an atom which has lost one electron 

 — the " ionised " atom, as it is termed — is what we have 

 called the spark spectrum, and the enhanced lines are the lines 

 produced by passages from one to another of the new orbits 

 made possible by the resultant positive charge on the atom. 

 Almost daily, fresh evidence pours in, supporting on one side 

 or another this remarkable conception of atomic structure, 

 and though at present it seems difficult to deduce from it the 

 facts of chemical combination, no contradiction is involved, 

 and it may well be that, with fuller understanding of its implica- 

 tions, and perhaps with some modifications of its details, its 

 scope will soon be as striking as its success in the minutiae of 

 spectrum phenomena. In any case, it has pointed the way 

 to more than one discovery, and in fertility of suggestion and 

 guidance in clear thinking, it leaves the spectroscopist little to 

 desire. 



Keats once complained that Newton had destroyed the 

 mystery of the rainbow by reducing it to the prismatic colours. 



