IT WAS an awfully good day in the White House yesterday for both Ronald Reagan and Mikhail Gorbachev. Beaming, they signed a missile treaty, and before and after they pursued a longer agenda that could produce, in the next few months if not in the next few days, broader arms-control agreements and perhaps more. The roller-coaster history of Soviet-American summits cannot be ignored, but neither can the apparent congruence of atmospherics and substance this time around.

Three years under negotiation and public debate, the treaty eliminating medium- and shorter-range missiles -- about 4 percent of the two sides' arsenals -- has a familiar, old-shoe quality. It pioneers by eliminating whole classes of missiles, by compelling disproportionate Soviet cuts to reach an equal floor and by sanctioning stunning on-site inspection. Although Mr. Reagan was too gracious to say so yesterday, for Americans the INF treaty marks a political triumph in holding the alliance together under a threat of Soviet nuclear intimidation. For the Soviets the setback is eased by seeing withdrawal of the fast, accurate missiles trained on Kremlin targets that the United States had deployed in response to their SS-20s.

Some conservatives, especially those fearful of the idea of arms control, find grounds for concern in the verification provisions and in Europe's questions about what comes next. There are grounds for concern but not, we think, for rejection of the treaty. The verification provisions need to be scrupulously applied and enforced -- and extended and deepened in subsequent arms-reduction negotiations. Europe's questions lead logically to Soviet-American talks on strategic arms in which Europe will be consulted and to East-West talks on conventional and chemical arms in which Europe will take part.

It appears that while certain ''reservations'' and ''understandings'' may be pasted on the INF treaty in the Senate, the text will not be laden with amendments requiring renegotiation with Moscow. Sen. Bob Dole's movement toward support of the treaty, late and awkward though it is, indicates the way the wind is blowing. It is a fair wind, and what Mr. Reagan and Mr. Gorbachev may yet do in and after the summit could make it stronger.