But  again  we  must  not  be  too  sure  in  either  case,
because  appearances  are  often  deceptive.  Have
you  ever  noticed  a  train  coming  nearly  straight  at
you  from  a  long  way  off?  When  you  are  waiting
at  a  station  sometimes  the  lines  curl  round  a  corner,
so  that  you  do  not  see  the  train  till  it  is  pretty
close;  but  sometimes  they  run  straight  for  a  long
distance  and  then  you  can  see  your  train  coming
from  a  long  way  off.  It  looks  no  bigger  than  a  toy
train,  and  moreover  it  seems  to  be  quite  still.  You
know  that  it  is  coming  towards  the  station  (at  least
you  hope  so  if  it  is  the  train  you  want  to  join),  but
you  cannot  see  it  move  because  it  is  coming  at  you
end-on.  It  might  even  be  going  the  opposite  way
for  all  you  can  tell  by  looking  at  it.  Similarly,  if
a  star  seems  to  us  to  remain  nearly  still,  we  must  not
be  too  hasty  in  thinking  that  it  has  very  little
movement  of  any  kind,  and  therefore  it  is  very
far  away,  for  it  may  be  coming  directly  at  us,  or
going  directly  away  from  us;  by  merely  watching


THE   STARS  271

it  we  cannot  tell.  Fortunately,  a  beautiful  method
has  been  discovered  by  which  we  can  determine
whether  this  is  so — whether  the  star  is  coming
towards  us,  and  how  fast  it  is  approaching.  Halley
showed  us  how  to  watch  the  stars  moving  across
our  line  of  sight,  and  another  great  Englishman,
Sir  William  Huggins,  showed  us  how  to  find  out
with  the  spectroscope  whether  they  are  coming
end-on  to  us,  or  moving  away  from  us  :  and  I  want
now  to  say  something  of  how  he  did  it.
