plane  between  the  two  media,  unless  it  falls  exactly  per-
pendicularly on  this  plane.  If  it  pass  into  a  denser
medium,  it  is  refracted  toward  a  perpendicular  to  this  plane  ;
if  into  a  rarer  medium,  it  is  refracted  from  the  perpendicu-
lar. The  angle  GDO  (Fig.  262)  is  called  the  angle  of  in-
cidence; FDN,  the  angle  of
refraction  ;  and  EDF,  the  an-
gle of  deviation.
279.  Cause  of  Refraction.
