required  to  start  the  lami)  the  current
is  turned  on  and  the  lamp  is  shaken
or  tipped  mitil  the'two  mercury^sur-
faceS  come  momentarily  into  contact,
that  the  tube  B  is  completely  exhausted.  A  large  current
should  not  be  used,  as  the  lamp  becomes  too  hot ;  cooling  it
'in  water  causes  a  condensation  of  mercury  upon  the  walls  of
the  tube  so  that  no  light  can  escape.  Lummer  ha§  modified
the  apparatus  so  as  to  overcome  this  defect.  ^

Barnes  ^  has  devised  a  very  convenient  form'  of  lamp,  which
is  shown  in  Fig.  145.     Through  the  neck  D  of  the  globular
vessel  A,  which  is  about  800  c.c.  in  volume,  passes,  the  iron
tube  E,  of  diameter 4 13  mm.     Along  the  ceritre  of  this  passesa
the  porcelain  tube  F,  8  mm.  in  diameter.     This  porcelain  tylje,-
is  connected   with  a  glass  tube,  which  in  turn  is  connoted
with, a  large  rubber  tu|)e.     Mercury  fills  the  space  between  the :
porcelain  and  iron  tubes,  as  well  as  the  porcelain,  glass,  and
rubber  tubes  attached.     The  electric  wires  are  attached  one  to
the  iron  tube  E  and  the  other  to  the  mercury  in  the  porcelain

'  Ann.  der  Phys.,  47.  767  (1892)  ;  and  S8.  73  (1896).
'  Astrapiys.  Journ.yX^.  190  (1904).,


'  Fig.  144.
It  should  be' remarked


396


SPECTROSCOPY


Fig.  145.
