[metapost] Why I can't see the text

[谢超]

    I've just typed in a simple metapost file, for example test.mp:

beginfig(1);
    dotlabel.lft(btex $O$ etex, (10cm, 10cm));
endfig;
end;

    The I run mpost but got nothing to show.
    I type in the command "gs test.1", get the following error messages:

ERROR: /undefined in cmmi10
Operand stack:
   (O)
Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--
--nostringval--   2   %stopped_push   --nostringval--
--nostringval--   --nostringval--   false   1   %stopped_push   1   3
 %oparray_pop   1   3   %oparray_pop   1   3   %oparray_pop   1   3
%oparray_pop   .runexec2   --nostringval--   --nostringval--
--nostringval--   2   %stopped_push   --nostringval--
Dictionary stack:
   --dict:1130/1686(ro)(G)--   --dict:0/20(G)--   --dict:102/200(L)--
Current allocation mode is local
Current file position is 277
ESP Ghostscript 815.04: Unrecoverable error, exit code 1

And the content of the test.1 is:

%!PS
%%BoundingBox: 279 273 288 281
%%HiResBoundingBox: 279.5265 273.65671 287.40251 280.46451
%%Creator: MetaPost 0.901
%%CreationDate: 2008.05.06:2031
%%Pages: 1
%*Font: cmmi10 9.96265 9.96265 4f:8
%%BeginProlog
%%EndProlog
%%Page: 1 1
279.5265 273.65671 moveto
(O) cmmi10 9.96265 fshow
showpage
%%EOF

Somebody told me maybe there's no cmmi10 fonts for postcript, so I
copy the cmmi10.tfm to /usr/share/ghostscript/fonts/ where ghostview
will search fonts file. But the problem still remains.

Could some one tell me what to do?  Thank you very much!