48  M.  F.  Zollner  on  the  Spectroscopic  Observation 
set  themselves  the  problem  of  turning  to  account  in  the  service 
of  astrophysics  the  means  so  very  liberally  placed  at  their 
disposal.  The  following  communications  on  the  spectroscopic 
observation  of  the  solar  rotation,  and  Dr.  VogePs  investigations 
on  the  aurora  borealis*,  may  be  regarded  as  the  firstfruits  of 
these  meritorious  endeavours. 
For  the  object  intended,  the  reversion-spectroscope  had  re- 
ceived some  alterations,  which  had  reference  partly  to  its  own  con- 
struction, and  partly  to  the  manner  of  fixing  on  the  Bothkamp  re- 
fractor. These  were  not  quite  finished  in  time  for  my  departure; 
so  that  I  was  obliged  to  have  the  instrument  sent  after  me.  It 
arrived  at  Bothkamp  on  the  21st  of  May,  and  was  immediately  at- 
tached to  the  refractor.  Unfortunately, during  the  three  dayswich 
still  remained  for  my  stay  there,  the  weather  was  unfavourable. 
Only  in  the  evening  of  the  2nd  of  June  did  a  break  in  the  clouds 
when  the  sun  was  near  the  horizon  permit  at  least  one  trial  to 
be  made.  While  I  looked  through  the  spectroscope,  Vogel,  with 
the  aid  of  the  finder,  directed  the  refractor  upon  various  parts  of 
the  sun's  disk.  I  observed  a  small  displacement  of  the  sodium- 
lines,  which  had  been  brought  to  coincidence  in  the  two  spectra, 
and,  without  knowing  any  thing  of  the  position  of  the  slit  on  the 
solar  disk,  always  answered  VogePs  questions  about  the  direc- 
tion of  the  displacement  in  a  manner  corresponding  to  the  situa- 
tion. But  after  the  lapse  of  a  few  minutes  the  sky  again  became 
covered;  so  that,  substantially,  I  returned  without  having  accom- 
plished my  purpose,  and  left  my  instrument  with  MM.  Vogel 
and  Lohse  for  further  investigations  under  more  favourable 
conditions. 
To  my  joy,  after  a  few  days  I  received  a  letter,  dated  June  9, 
1871,  in  which  Dr.  Vogel  gives  me, 
"Today,  as  briefly  as  possible,  the  intelligence  that  we  (Dr. 
Lohse  and  I)  yesterday  positively  and  repeatedly  saw  the  dis- 
placement of  the  lines  by  the  rotation  of  the  sun,  by  means  of 
the  reversion-spectroscope.  With  the  present  arrangement  of 
the  apparatus,  it  is  hardly  possible  to  execute  measurements  with 
any  certainty ;  above  all,  the  two  spectra  must  be  perfectly  equal 
in  brightness,  and  the  slit  must  be  as  fine  as  it  is  anyhow  possible 
to  make  it,  since  in  such  delicate  measurements  the  cross  lines 
are  too  coarse." 
A  second  letter,  dated  June  14,  contains  the  following: — 
"Since  with  your  spectrum-apparatus  (as  I  have  already  in- 
formed yon)  the  displacement  of  the  lines  by  the  rotation  of  the 
sun  was  seen,  with  my  own  I  have  not  only  seen  a  displacement, 
but  estimated  its  magnitude,  by  using  five  strongly  dispersing 
prisms  arranged  in  a  circle,  having  previously  directed  them 
*  Berichte  der  Kon.  Sachs.  Ges.,  I.  c. 
