5.  Similar  planes.  Similar  armies. — The  lateral  faces  of  a  square
prism  (Fig.  2,  p.  15)  are  equal  ana  have  dke  relations  to  the  axes,  and
hence  they  are  said  to  be  sinUiar.  Solid  angles  are  similar  when  the
plane  angles  are  equal  each  for  each,  and  the  enclosing  planes  are  sev-
erally similar  in  their  relations  to  the  axes.
6.  Truncaied.  BeveUed. — An  edge  of  a  crystal  is  said  to  be  trun-
ecUed  when  it  is  replaced  bv  a  plane  equally  inclined  to  the  enclosing
planes,  as  in  Fig.  13,  p.  20 ;  and  it  is  bevelled  when  replaced  by  two
planes  equally  inclined  severally  to  the  adjoining  faces.  Only  edges
that  are  formed  by  the  meeting  of  two  simuar  planes  can  be  truncated
or  bevelled.  The  angle  between  the  truncating  plane  and  the  plane
adjoining  it  on  either  side  always  equals  90"  plus  half  the  interfacial
angle  over  the  truncated  edge.  When  a  rectangular  edge,  or  one  of
90",  is  truncated,  this  angle  is  accortlingly  185"  (=  90**  +  45') ;  when
an  edge  of  70%  it  is  125"  (=  90"  +  85") ;  when  an  edge  of  140",  it  is
160"  (=  90"  +  70").
7.  Zone. — A  zone  of  planes  includes  a  series  of  planes  having  the
edges  between  them,  that  is,  their  mutual  intersections,  all  parallel.
