The most innocent of challenges has been made but Jim Corcoran already is going through motions. He pulls at the laces of his frazzled Pro-Keds, tightens his face and fixes his stare. a

"I'm six feet tall."

Corcoran looks for the faintest sign of doubt and finds it.

"People think I'm lying about my size, but look. . ." He bolts out of his chair and stands as if poised to pledge allegiance. "You see? I'm at least 5-11 3/4."

At least. Until the next doubter appears. Corcoran can relax. He's found a believer.

Jim Corcoran's crusade to get people to believe in him began the moment he came to Georgetown University where he was dead set on playing varsity football, basketball and baseball.

This fall, the crusade is "paying off in spades," says his football coach, Scotty Glacken. As free safety for the Hoyas, Corcoran had seven interceptions in their first five games. He's riding the crest of a wave and he wants people to notice.

"Jimmy Corcoran is a very intense young man," says Glacken. "He's intense off the field, in practice, everywhere.Everything's a challenge."

Corcoran shrugs off the interceptions. Even the one he ran back 55 yards for a touchdown against St. Peter's. He shrugs off the 48-yard punt return for a touchdown against Duquesne and a 40-yard return against Galluadet. He shrugs off two 30-yard yard field goal he booted against St. Peter's. That's all past.

"I can't rest on that stuff," he says, "I've got to keep going and not let up."

At Georgetown, where football players don't receive scholarships it's rare to hear football players talk with such zeal. But Corcoran wants more out of college football than his teammates do. He wants it all. . . the pros.

"I've always wanted to be a pro athlete," Corcoran says. "Everyone thinks I can't do it but my family is there. All I know is that my ma and pa think I can do it and that's enough for me right there."

Corcoran realizes he would get more notice at another school. Coming out of Georgetown Prep in Potomac, Corcoran "had other choices. I could've gone to the Ivy League, Yale, but they have that freshman ineligibility and I wanted to play. I could've gone to Stanford but they didn't guarantee a scholarship, so I'm here on financial aid. My pa says it doesn't matter where you are. If you're good, you get noticed."

Some people around Georgetown suspected Corcoran might transfer.

"With seven interceptions in five games, I thought he was gone," said Howie Kopech, of the Georgetown sports information office. "But he's very close to his parents and I think he wants to stay around here."

Corcoran admits his family is the bottom line. Especially his mother, who has been disabled by multiple sclerosis."She comes to every game in a wheelchair," says Corcoran quietly. "She thinks I can make it in the pros."

After every game, before heading to the locker room, Corcoran huddles with his mother and they discuss the game. One look at No. 17 sitting on his helmet next to his mother and it's clear Corcoran plans to make it work at Georgetown.

"Hell no," says Glacken. "Jimmy knows the value of a Georgetown education. He won't transfer. Ten years from now I think football may still be important to Jimmy but his Georgetown education will be much more important."

Corcoran is in the Georgetown business school but, dispite Glacken's viewpoint Corcoran hopes football is the most important thing in his life in 10 years.

"I'll be disappointed if I'm not a pro athlete but I don't plan on being disappointed," he says. "I'm not saying I'm going to make it but. . ."

He nods.

"I'm gonna make it," he's saying to himself.

Glacken says, "It's far too early to tell about the pros. Ask me in the second half of his senior year. Jimmy is a great athlete, he's way ahead of himself, but I think he has a long way before he's a great college football player. One thing he has for sure is the attitude to make it."

Corcoran's atitude is right out of Steve Garvey's Familiar Quotations. "You gotta give a 100 percent . . . no, a 110 percent. That extra 10 percent gets you going." And, "It's nice to have good things happen to you, but the team comes first." And "Winning is what you're out there for."

The All-American values roll off Corcoran's tongue with unwavering convicton. But Corcoran's confidence rarely wavers.

"He's pretty cocky," says sports information director Jim Marchiony. "But everyone seems to like him. With all the fuss I don't think people resent him."

Corcoran has junior varsity basketball on his mind for the winter and varsity baseball for the spring. He's sure of making it big in baseball ("I'll play anywhere") but more reserved about basketball. "The varsity has great players. They're big and they're good. But I play taller than I am."

Corcoran hints that it will all come down to football.

"I'm up to 185 pounds," Corcoran says. "I'm real strong."

He curls his arms. The 300-pound barbell he envisions in his hands pulls smoothly up to his chin. "In two years, I'll be up to 195. I'll be a rock. sWhen anyone doubts me now, I just say, 'I'll see you in three years.'"