It  had  always  been  a  formidable  objection  to
the  Copernican  theory  that  this  appearance  of  the
planets  had  not  been  observed.  The  author  of  that
theory  had  endeavoured  to  account  for  this,  by
supposing  that  the  rays  of  the  sun  passed  freely
through  the  body  of  the  planet;  and  Galileo  takes
occasion  to  praise  him  for  not  being  deterred  from
adopting  the  system  which,  on  the  whole,  appeared
to  agree  best  with  the  phenomena,  by  meeting  with
some  appearances  which  it  did  not  enable  him  to
explain9.  Yet  while  the  fate  of  the  theory  was  yet
undecided,  this  could  not  but  be  looked  upon  as  a
weak  point  in  its  defences.
The  objection,  in  another  form  also,  was  embar-
rassing alike  to  the  Ptolemaic  and  Copernican  sys-
tems. Why,  it  was  asked,  did  not  Venus  appear
four  times  as  large  when  near  her  perigee,  as  when
near  her  apogee?  The  author  of  the  epistle  pre-
9  L.  U.  K.  Life  of  Galileo,  p.  35.
SEQUEL  TO   COPERNICUS.  417

fixed  to  Copernicus's  work  had  taken  refuge  in  this
argument  from  the  danger  of  being  supposed  to
believe  in  the  reality  of  the  system ;  and  Bruno  had
attempted  to  answer  it  by  saying,  that  luminous
bodies  were  not  governed  by  the  same  laws  of  per-
spective as  opaque  ones.  But  a  more  satisfactory
answer  now  readily  offered  itself.  Venus  does  not
appear  four  times  as  large  when  she  is  four  times
as  near,  because  her  bright  part  is  not  four  times
as  large,  though  her  visible  diameter  is ;  and  as  she
is  too  small  for  us  to  see  her  shape  with  the  naked
eye,  we  judge  of  her  size  only  by  the  quantity  of  light.
