The Supreme Court's flame thrower has been extinguished for the summer, but not before capital punishment again caused the Constitution to get singed.

The court has hitherto said that such punishment is not inherently "cruel and unusual," but has been unable to suppress its instinct for fine-tuning. Now the court has held, 5-4, that capital punishment would be "cruel and unusual" if inflicted on Earl Enmund, 50, who planned and participated in a robbery during which a Florida couple, ages 86 and 74, were murdered.

The couple resisted with a gun, and evidently were killed by two of Enmund's friends, his role perhaps being confined to driving the getaway car. But under Florida's felony murder rule, a person can be sentenced to death if he was present at, and abetted, a robbery during which murder occurred. Enmund claimed that the death penalty would be unconstitutionally disproportionate because he personally neither took, attempted to take nor intended to take life.

Justice Byron White, for the majority, recalled that when the court held in 1977 that the death penalty is excessive for rape, it cited "objective factors," including statistics about legislative actions, to discern society's "evolving standard" concerning punishment. By making various narrow distinctions, White is able to argue that only nine jurisdictions allow capital punishment for precisely the sort of participation in a felony murder that Enmund is guilty of--at least as White characterizes Enmund's participation.

For the minority, Justice Sandra O'Connor stops short of the old rebuke that White uses statistics the way a drunk uses lampposts--for support, not illumination. But she says his method of counting states cannot hide the fact that of 35 states that have the death penalty, 21 permit it for felony murders even when the defendant did not commit the homicidal act and had not intended to kill. Three additional states require some finding about mental state (such as "extreme indifference to life"), but not the intent to kill that the court has now made a constitutional requirement for imposition of the death penalty. So in the two- thirds of the states that impose the death penalty, the person sentenced need not have killed or have had an intention of killing. Hence the majority is wrong to say the "society" has conclusively rejected capital punishment for involvement in felonies that result in murders.

White emphasizes the principle that causing death intentionally should be punished more severely than participating in a crime during which someone else kills. O'Connor accuses White of "disingenuously" characterizing Enmund as a mere "robber" playing a "relatively minor" role in the crime, whereas in fact he was initiator and planner. Thus he is at least partly responsible for the murders. Hence it is unreasonable to say that this case, like the case involving a death sentence for rape, involves punishment disproportionate to the kind of harm done.

White emphasized that the death penalty for accomplices in felonies involving murders is rare: Enmund is one of just three such persons sentenced to death in 25 years, and the infrequency of the sentence makes its deterrent effect negligible.

But White makes a startling use of a statistic to make his point. He says the felony murder rule cannot be a significant deterrent because the likelihood of a killing during a robbery is not substantial. True, killings occur during only about one-half of one percent of all robberies. But in 1980 there were 548,809 robberies: one- half of one percent came to 2,361 persons killed.

That is more than have been killed in the violence in Northern Ireland in the last 12 years. If an infectious disease killed 2,361 persons in a year there would be something like panic. There was when polio was killing 300.

All the court can do concerning the deterrent value of the felony murder rule, O'Connor says, is speculate. But that is the sort of judgment about which legislatures should be granted broad discretion.

For centuries, various societies have used various versions of the felony murder rule to say to participants in armed robberies: we are serious--deadly serious --about stopping you. Now the court has --what? discovered?--that the Constitution says that capital punishment is disproportionate unless a murderer had a particular intent. But, O'Connor wonders, what principle of constitutional reasoning justifies disqualifying the intent to commit armed robbery knowing that it involves risk of death for the victim?

The answer is: no constitutional principle requires what the majority did. What the majority did was not constitutional law, it was legislation..