The fonts normally used by LATEX are from the Computer Modern Roman
font family designed by Donald Knuth. Some people prefer the Times
Roman family of fonts, which are available on the Apple LaserWriter
printers. They are obtained with the ps document-style option.
Currently, this option does not work with the 12pt style option.
Also, fonts from the Times Roman family are available only for the
default \rm style and for the \it, \bf, and
\sf styles. Other styles use the ordinary CMR fonts.
TEX can use only the CMR fonts in math mode, so ${\rm foo}=3$
produces ``foo'' in CMR font, which is probably not what you want.
Instead, use $\mbox{\rm foo}=3$, which typesets ``foo'' in Times
Roman because TEX is not in math mode inside the \mbox. No
one will notice that ordinary math symbols like α or x are
typeset in a CMR font rather than a Times Roman font.
A dvi file produced with the ps option can be printed only on the Apple LaserWriter using the aptex program. Type the Ultrix command man aptex to find out how to run this program. You can print any dvi file with aptex, but it is slower than iptex and doesn't do a very good job printing the standard CMR fonts. So, use aptex to print only dvi files created with the ps option.
The ps option causes LATEX to use more fonts than usual, so there's a good chance that you'll run TEX out of font space if you use lots of different fonts.