582

3571

38

581

3609

20

S83

3629

562  SPECTROSCOPY

same  element  as  well  as  the  actual  expression  of  the  lines
in  series.
Two  very  important  observations,  one  by  Hartley  and  the
other  by  Liveing  and  Dewar,  must  first  be  mentioned.  In  the
first  place,  Hartley  '■  found  that  if  the  lines  in  a  spectrum  are
expressed  in  oscillation  frequencies  and  not  in  wave-lengths,
then  the  differences  between  the  components  of  the  doublet  or
triplet  are  the  same  for  all  the  doublets  or  triplets  in  the  same
spectrum.  Hartley  only  examined  the  triplets  in  the  spectra
of  magnesium,  zinc,  and  cadmium,  and  as  an  example  three
triplets  of  the  zinc  spectrum  may  be  given — the  oscillation
frequencies  are  only  given  to  four  figures—

Diff.
1  2079  910        2989
Diff.      39  39

2  21 18  910       3028
Diff.      18  18

3  2136  •  910        3046

The  numbers  i,  2,  and  3  refer  to  the  first,  second,  and
third  members  of  the  triplets ;  it  will  be  seen  that  the  differ-
ence between  the  first  and  second  member  is  about  39,  and
between  the  second  and  third  it  is  about  18.
The  second  discovery,  by  Liveing  and  Dewar,''  was  that  of
the  existence  of  homologous  series  of  lines,  although  they  did
not  find  the  law  of  the  series.  They  established  also  the
existence  of  a  series  of  lines  which  are  sharp,  and  a  similar
series  of  diffuse  lines.
