528 Professor John W. Nicholson [May 2^ 



same transfer quite definitely, though on a smaller scale, and the 

 effect in these series is more closely confined to enhancement of the 

 central intensity. 



The most striking enhancements produced by the condensed 

 discharge in hehum occur with the lines A 4472 and A 4388, which 

 are precisely the helium lines apt to be found abnormally strong in 

 the spectra of some of the planetary nebulae. Some other experi- 

 ments we have made, on the spectrum of helium at very low pressure, 

 indicate that these lines, together with the line A 5015 more 

 frequently quoted, are the strongly enhanced lines also in these 

 circumstances. If the two sets of circumstances occur together, 

 A 5015 is not especially strong, but the others are enhanced for both 

 reasons. We have in fact been able to demonstrate that the 

 peculiar " nebular " spectrum of helium could be produced in the 

 laboratory by a combination of the condensed discharge with an 

 extremely low pressure. 



I shall not discuss the spectra of gases as dependent, in their 

 intensity relations, on pressure. The time required would be pro- 

 hibitive, and my object is to indicate the range of work now open to 

 precise investigation, rather than to give any complete account of the 

 phenomena which the method has yet indicated or elucidated. One 

 remark must, however, be made in connection with high-pressure 

 spectra. We investigated the intensity distribution in a helium tube 

 at the extraordinary pressure of 42 mm. Except for the trace of 

 hydrogen which came out of the electrodes during the discharge the 

 helium was pure. Yet the hydrogen spectrum was nevertheless pre- 

 dominant on the plate, and fourteen members of the Balmer series,- 

 instead of the usual seven or eight, could be seen visually as very 

 sharp lines. This plienomenon incidentally cannot be reconciled 

 with the current quantum theory of the hydrogen spectrum— perhaps 

 not an unexpected fact to those conversant with the hydrogen 

 spectrum. No spectroscopist can in fact accept a theory which can 

 give no hint of the origin of the so-called '' secondary spectrum " of 

 hydrogen, known to arise mainly from the atom, and, in the laboratory 

 at least, the most important and extensive part of the spectrum. 

 The elucidation of this spectrum is in many ways the most funda- 

 mental problem of physics, and far more fundamental than the 

 Balmer series problem. No atomic theory as yet has begun to inter- 

 pret any of this spectrum, and several have given a theoretical account 

 of the Balmer series. 



Many of the new problems of interest, which the possession of an 

 accurate method of intensity determination in spectra enables us to 

 attack, are mainly of astrophysical importance. There may be varia- 

 tions of intensity in the Fraunhofer lines accompanying other more 

 readily perceived solar phenomena, for example, but of more urgent 

 importance is the need for a series of photographic registers of the 

 intensity across the whole spectrum of a new star at different stages 



