 SCX FANG</strong>
commands. The <strong>FOB</strong> command, in many cases, also causes
a central or <em>chief</em> ray to be traced. This ray starts at the
specified object point, is iteratively aimed so as to pass through the
center of the current reference surface, and then proceeds through the
rest of the current lens. The <strong>RAY</strong> command is used to
specify and trace all general <em>non-chief</em> rays by specifying
the relative Y and X coordinates of the ray in the current reference
surf