WHEN UNIVERSITY POLICE CHIEF Saul L. Chafin officially departed Friday to take a similar post at Vanderbilt University. Harvard lost the best cop it's ever had. University administrators, and more importantly the police officers themselves, credit Chafin with a renaissance in the department during his five-year term.

Through his leadership, Chafin transformed a force filled with union antipathy toward his predecessor and stymied by low morale into an efficient unit capable of making Harvard secure in its unique urban setting. Crime statistics indicate that the University is a safer place now than it was five years ago.

Two of Chafin's principal innovations included sending patrolmen through the State Police Academy's full 12-week training course and establishing a sensitive crime unit to investigate sex offenses.

As recognition of the department's caliber, Harvard officers were deputized in Middlesex and Suffolk Counties, allowing them to make arrests off Harvard property--the only campus force so designated in metropolitan Boston.

Harvard must be extremely cautious in filling the hole Chafin left behind. The wrong person in the department's top post could easily cause it to stumble in the strides it has made.

Vice President Daniel Steiner '54 made the right first move by appointing University Police Capt. Jack W. Morse, Chafin's second-in-command, as interim security director. Morse is a fine administrator who can keep the department running at a high standard.

But Harvard should not use Morse's presence as an excuse for postponing a decision on a new police chief. The department cannot run on momentum forever, and members of the search committee Steiner put together to consider possible replacements said recently that very little substantive work has been done.

Steiner's next step must be to select at least one student representative for the search committee. Associate deans may be excellent assessors of managerial skill, but Chafin's successor will need more than that to do the job well. He'll have to deal with students at least as well as he handles annual budgets and patrol schedules.

The obvious choice for a spot on the search committee is a member of the student group that met regularly with Chafin to discuss security issues. Any one of them would be able to raise student concerns in the selection process.

A worthy goal the search committee should consider is replacing Chafin, a Black, with another minority student director. Boston's deplorable racial climate is well known and, in general, minorities traditionally have been the target of police harassment and violence. The presence of a minority police chief would be a reassurance to the University community that the Harvard department will not engage in such practices.

In addition, Chafin had pursued affirmative action in hiring, and such a policy should hold true all the way to the top man.

The University has a responsibility to protect its faculty, staff, and students. It could go a long way toward that end by picking a new police chief equal to his predecessor.