By  the  physical  hypothesis,  he  meant  the  doc-
trine that  the  time  of  a  planet's  describing  any
part  of  its  orbit  is  proportional  to  the  distance  of
the  planet  from  the  sun,  for  which  supposition,  as
we  have  said,  he  conceived  that  he  had  assigned
physical  reasons.
The  two  last  hypotheses  came  the  nearest  to
the  truth,  and  differed  from  it  only  by  about  eight
minutes,  the  one  in  excess  and  the  other  in  defect.
And,  after  being  much  perplexed  by  this  remaining
error,  it  at  last  occurred  to  him11  that  he  might
take  another  ellipsis,  exactly  intermediate  between
the  former  one  and  the  circle,  and  that  this  must
give  the  path  and  the  motion  of  the  planet.  Making
this  assumption,  and  taking  the  areas  to  represent
the  times,  he  now  saw12  that  both  the  longitude
and  the  distances  of  Mars  would  agree  with  ob-
servation to  the  requisite  degree  of  accuracy.  The
rectification  of  the  former  hypothesis,  when  thus
stated,  may,  perhaps,  appear  obvious.  And  Kepler
informs  us  that  he  had  nearly  been  anticipated  in
this  step.  (c.  55.)  "David  Fabricius,  to  whom  I
had  communicated  my  hypothesis  of  cap.  45,  was
able,  by  his  observations,  to  show  that  it  erred  in

11  De  StelM  Martis,  c.  58  12  Ibid.  p.  235.
VOL.  I.  G  G


450        HISTORY  OF  FORMAL  ASTRONOMY.
