246 SCIENCE PROGRESS 



plate are read off under a travelling microscope, which is usually 

 graduated to read to -ooi mm. It takes some practice to 

 make accurate micrometer settings, and those made by an 

 observer new to the work are usually unrehable for the first few 

 months. Wave-lengths determined in this way should be 

 correct to a few hundredths of an Angstrom unit. 



Over 1,200 lines were measured in the secondary spectrum, 

 and it would obviously require a jig-saw expert of superhuman 

 ability to distinguish, by mere inspection, the related lines in 

 such a tangle. It is fortunately possible to simplify the problem 

 by studying the physical behaviour of the lines. It has been 

 found that all the lines belonging to the same series in the 

 spectrum of an element show similar Zeeman effects, are broad- 

 ened to about the same extent by a condensed discharge, and 

 are enhanced or weakened together by changes in the conditions 

 of production, while the behaviour of the different series is quite 

 distinct. Since this fact was observed it has been employed 

 in the discovery of series in complicated spectra. Another 

 criterion for related lines is that the intensities in a band or 

 series alter continuously from member to member without any 

 sudden break ; but this is of more assistance in testing suspected 

 regularities than in their discovery in the first place. The 

 lines of the secondary spectrum can certainly be divided into 

 several groups of lines of similar behaviour. The intensity 

 distribution in the spectrum is, for example, completely upset by 

 the introduction of helium into the hydrogen vacuum tube. 

 Most of the lines are weakened, but a few are very much 

 enhanced, and some totally new ones — undoubtedly due to 

 hydrogen^ — make their appearance. It may be expected that 

 some regularity will be found from a study of the small number 

 of lines which are enhanced. Similar groups were distinguished 

 by intensity changes in the spectrum when the pressure in the 

 discharge tube was altered from about o-i mm. to 50 mm. of 

 mercury, and by changes when a condenser and small spark-gap 

 were introduced into the exciting circuit. Previous work by 

 Dufour showed that some of the lines exhibited a Zeeman 

 effect, while others were quite unaffected in a magnetic field ; 

 it is also known that some of the lines show a Stark effect. 



Even with all these indications to assist, the structure of the 

 secondary spectrum remains an open question. The only 

 known regularities are still the few constant wave-length 

 differences which Fulcher observed among some of the stronger 

 lines, several years ago. One difficulty is of course that we do 

 not know exactly what kind of regularity to expect. There is 

 no doubt that a solution of the problem will rapidly be fol- 

 lowed by important advances in our knowledge of molecular 

 constitution. 



