56.  A  model  will  be  required  or  admitted  as  a  part  of
the  application  onl,y  when  on  examination  of  the  case  in
its  regular  order  the  primary  examiner  shall  find  it  to
be  necessary  or  useful.  In  such  case,  if  a  model  has  not
been  furnished,  the  examiner  shall  notify  the  applicant
of  such  requirement,  which  will  constitute  an  official  ac-
tion in  the  case.  AVhen  a  model  has  been  received  in  com-
pliance with  the  official  requirement,  the  date  of  its  filing
shall  be  entered  on  the  file  wrapper.  Models  not  re-
(juired  nor  admitted  will  be  returned  to  the  applicants.
When  a  model  is  retpiired,  the  examination  may  be  sus-
pended until  it  shall  have  been  filed.
57.  The  model  must  clearlj-  exhibit  ever\^  feature  of
the  machine  which  forms  the  subject  of  a  claim  of  inven-
tion, but  should  not  include  other  matter  than  that  cov-
ered by  the  actual  invention  or  improvement,  unless  it
be  necessary  to  the  exhibition  of  the  invention  in  a  work-
ing model.
58.  The  model  must  be  neatly  and  substantially  made
of  durable  material,  metal  being  deemed  preferable ;   but

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when  the  material  forms  an  essential  feature  of  the  in-
vention, the  model  should  be  constructed  of  that  mate-
rial.
59.  A  working  model  ma.y  be  required  if  necessary  to
enable  the  office  fully  and  readily  to  understand  the
precise  operation  of  the  machine.
