The  visibility  curve  of  the  red  hydrogen  line  is  shown  in
the  full  curve  in  Fig.  122  (^);  the  dotted  curve  represents —


V  =  2     ^9'  cos  ^.
30


The  visibility  curve  is  therefore  practically  the  same  as  that
calculated  for  a  double  source  whose  ratio  of  brightness  is
equal  to  07.
Now,  D  in  the  formula  for  V,  or  the  period  of  the  curve,  is
inversely  proportional  to  the  distance  between  the  components
of  the  pair  of  lines,  this  distance  being  measured  as  hitherto
in  oscillation  frequencies.     We  may  therefore  put  the  distance

between  the  wave-lengths  of  the  two  lines  equal  to  j^.    The

formula  for  the  hydrogen  red  line  gives  D  =  30,  and  as
X  =  6-56  X  io~*  mm.,  we  have  for  the  separation  of  the  two
components —


(6-56  X  10-")°
'      30


=  0-14  A.U.
3i6


SPECTROSCOPY


St


Again,  the  formula  gives  A  =  ig,
whence  we  find  that —

o

8  =  o"oii5,  or  0*049  in  Angstrom  units.
Therefore  the  half-width  of  each  com-
ponent is  o"o49  A.U.  From  these
data  the  curves  in  Fig.  122  {a)  were
drawn,  the  full  curve  showing  the  distri-
bution of  the  intensity.
Similarly  for  the  blue  hydrogen  line
the  equation  of  the  visibility  curve  was
found  to  be

2L'         07

V  =  2    24»  cos  ~^,
20

so  that  here  again  is  a  double  line  with
10  :  7  as  the  ratio  of  intensity  of  the
components,  Xj  —  Xj  =  o'o8  A.U.  and
8  =  0-057  A.U.
