Justice has been done in the Oliver North case.

Now it's time to start tempering justice with mercy, short of a presidential pardon that would tend to deprive the conviction of the former Marine lieutenant colonel of whatever deterrent effect it may have.Though North was convicted Thursday on only three of the 12 criminal counts against him in connection with the Iran-Contra case, the verdict is anything but a moral victory for North.

If given the maximum penalty on all three counts, he could pay up to $750,000 in fines and spend as much as 10 years in prison.

Even if those penalties are moderated, North will already have been punished severely by the abrupt, premature end to his military career and by the lasting damage done to the reputation of this proud man.

At the same time, it's hard to see how the jury could have treated North much more gently. Before the case went to the jury more than 12 days ago, North spent his last moments on the witness stand blaming his problems on others, admitting he repeatedly lied to this nation's lawmakers, and claiming that he did not know that lying to Congress was a crime.

Nonsense. As the prosecution brought out, Marine officers are not trained or expected to lie. Besides, both the honor codes of the military academies and the Uniform Code of Military Justice require that members of the armed services must disobey an illegal order.

In any event, North has now been convicted of illegally destroying documents, obstructing Congress, and accepting an illegal gratuity - a $13,800 home security system purchased with money from a Swiss bank account also holding proceeds from the sale of U.S. arms to Iran.

In deciding how severely North is to be punished, the court should keep in mind that the crimes for which North was convicted represent failures of judgment rather than failures of character. They were not the acts of a selfish, greedy man but of one who mistakenly thought his country was best served by keeping Congress in the dark and who was understandably concerned about the safety of his family and himself.

Though two presidents have described North as a hero, he is that only for his service in combat and not his misguided actions in the Iran-Contra affair. But he also is anything but Public Enemy No. 1. Now that the court has passed judgment on Oliver North, the nation will judge the court on the basis of how carefully it balances the conflicting demands of toughness and tenderness in rendering justice.

One final point: Thursday's verdict on North is anything but the last word on the Iran-Contra affair. Others face trial. Some members of Congress are talking about possible new investigations to determine whether the previous administration deceived congressional investigating committees.

Certainly, lieutenant colonels are not the only officials who should not be allowed to break the law in the name of patriotism. But it's hard to imagine a new investigation discovering much more than was uncovered by two previous probes. And though the North affair is widely described as Irangate, this situation clearly falls far short of the Watergate scandal.