of  the  Rotation  of  the  Sun.  51 
facility  for  all  spectroscopic  investigations,  without  its  being  ne- 
cessary, as  hitherto,  to  use  cross-threads,  points,  or  illuminated 
objects  for  the  determination  of  the  relations  of  position  of  the 
lines. 
The  requisite  disposition  of  the  observing-tube  of  any  spectro- 
scope can  be  secured  in  two  ways,  namely : — 1,  by  the  reversion- 
objective;  2,  by  the  reversion-ocular. 
1.  Description  of  the  Reversion- Objective. 
The  objective  of  the  observing-tube  is  divided  diametrically ; 
and  the  two  halves,  by  means  of  screws,  can  be  shifted  only  per- 
pendicularly to  the  line  of  division — that  is,  brought  nearer 
together  or  their  mutual  distance  increased.  In  front  of  one  of 
these  two  halves  a  rectangular  reflection-prism  is  placed,  move- 
able, in  such  wise  that  the  hypotenuse-surface  is  perpendicular 
to  the  plane  which  is  parallel  with  the  line  of  division,  and,  in 
the  normal  arrangement,  parallel  with  the  optic  axis  of  the  tele- 
scope. When  any  object  is  viewed  through  a  telescope  provided 
with  such  an  objective,  in  a  direction  perpendicular  to  the  line  of 
division  of  the  objective  it  appears  double.  On  the  one  hand,  it 
depends  on  the  dimensions  of  the  object,  on  the  other  on  the 
distance  between  the  two  halves  of  the  objective,  whether  the  two 
components  of  the  double  image  exactly  touch,  or  are  super- 
posed, or  are  separated  the  one  from  the  other.  But,  at  the 
same  time,  the  one  of  the  two  components  the  rays  of  which 
have  passed  through  the  reflection  -prism  is  inverted  relatively  to 
an  axis  perpendicular  to  the  line  of  division. 
Hence,  if  the  observing-tube  of  any  spectrum-apparatus  be 
replaced  by  a  telescope  with  such  an  arrangement,  and  the  latter 
be  placed  so  that  the  refracting  edges  of  the  reflecting  and  dis- 
persing prisms  are  parallel,  two  contiguous  spectra  with  the 
series  of  colours  in  opposite  directions  will  be  obtained  when  the 
distance  between  the  two  halves  of  the  objective  is  regulated 
accordingly.  As  with  non-parallel  rays  the  divergence  or  con- 
vergence is  altered  by  total  reflection,  in  such  cases  a  moveable 
half  of  a  lens  must  be  placed  in  the  observing-tube  in  order  to 
restore  equal  focal  distance  of  the  two  halves  of  the  objective. 
By  changing  the  direction  of  the  optic  axis  of  the  observing- 
tube  in  the  manner  which  is  usual  for  the  observation  of  different 
parts  of  the  spectrum  in  the  middle  of  the  field  of  view,  one  sees 
the  lines  of  the  two  spectra  move  through  the  field  in  opposite 
directions,  and  can  thus,  by  reading  off  the  angle  of  inclination, 
when  the  same  lines  in  the  two  spectra  coincide,  gather  their  po- 
sition— exactly  as  when  sights  are  used  in  the  field,  but  with 
double  the  amount  of  dispersion,  and  with  the  greater  accuracy  re- 
sulting from  the  principle  of  double  images.     Moreover  differential 
E2 
