Students in blue jeans and puffy down-filled coats filled very chair and standees spilled into the vestibule of Georgetown University's Dahlgrene Chapel at the noon mass yesterday.

It was Ash Wednesday and, like Christians throughout most of the world, the Georgetown students werre gathered to mark the beginning of Lent, the 40-day period of introspection and prayer preceding Easter.

"Lent is the time when we bring the images together - the image of what we think we are and the image that God has for us," the black-and-purple-robed priest told the student congregation.

Then the priest, the Rev. Michael Taylor, who is himself a graduate student in mathematics at the Jesuit university, invited the worshippers to come forward.

Dipping his thumb in a small brass pot, he made the sign of the cross on each worshipper's forehead with ashes.

Roman Catholics and some Protestant and Eastern Orthodox groups mark Ash Wednesday, the beginning of Lent, in this fashion, while nonliturgical Protestant churches do it with a simple prayer or preaching service.

Rigid rules of fasting and abstaining from eating meat throughout Lent have been relaxed for Roman Catholies. But the tradition of voluntarily giving up something for Lent persists.

Hartina Flournoy, a senior at Georgetown, said after yesterday's mass that she planned to "give up some of my personality traits - I'm going to try to be nicer to people."

Burke McCormick, also a senior, said he felt giving up something is "old-fashioned." He said that he would "try to be a little more Christian" during Lent.

Jeff Brewster, a sophomore, said that for him, Lent still meant "abstinence and fasting." He said he planned to do both, as well as spend time "doing something for others."

At the Vatican, Pope Paul VI, in his Lenten message, encouraged the faithful to perform acts or sharing and charity during Lent.

"The Christian finds joy in the many ways of showing love for one's neighbor, from alms-giving an serving individuals to collective contributions for the advancements of peoples who are at a material disadvantage," he said.

For Catholics in one part of the United States, Lent will be delayed. Because of the blizzard that swept Rhode Island earlier this week, Bishop Louis Gelineau has postponed Ash Wednesday observances until Sunday.