help wanted w. vtroff font change

[Jim Kempf]

I am trying to do font changes within vtroff using the troff .fp command.
vtroff tells me that it can't find the specified font in /usr/lib/font.
I could find no information in the Nroff/Troff Reference Manual or
accompanying documentation about what
legal font names are for the .fp command, nor anything in the vtroff
documentation about how to change a font within text. Furthermore,
when I tried to change the point size using the .ps command, vtroff
simply used the old point size for the type, but spread it out as
if it had used the new one.

Does anyone know how to do a font change within text being formatted
using vtroff? How to change point sizes in vtroff and have the letter
size also change?  I am using the ms macro package if that is any help.