Television made its debut in Parliament today, arriving a little late but carrying a lively and modern debate from the ancient halls of the House of Lords into the homes and offices of about 2 million viewers.

The live televising of a debate in Britain's upper house marked the start of a six-month experiment that could lead to cameras being admitted to the far more powerful House of Commons.

By the account of British commentators, the star of today's five-hour session was Harold Macmillan, who was Conservative prime minister from 1957 to 1963 and is now the Earl of Stockton. He will be 91 on Feb. 10.

Macmillan seized the opportunity to deliver an eloquent, frequently humorous, yet stern warning to Britain that it risked sinking "slowly and majestically . . . like a great ship" unless it faced up to the economic realities of what he called the third industrial revolution.

In his 20-minute speech, delivered without notes, Macmillan teased Conservative Prime Minister Margaret Thatcher for her monetarist policies. He called on his countrymen to emulate the United States and some other countries in spending money to create wealth and jobs in new technical and service industries.

Four or five years ago, he told the packed chamber and public galleries, "when the rather depressing reign of president Carter came to an end," America was in much the same situation as present-day Britain. The economy was depressed, unemployment was very high and there seemed to be not much hope, he said.

"Then President Reagan did a very wise thing. He dismissed all the academic economists in Washington," Macmillan said, to the enjoyment of many in the audience. "Happily for him and unhappily for us, the monetarists were the first to be exiled and they received the customary hospitality we always extend to refugees and they settled in Oxford, Cambridge, Whitehall and, it is rumored, even in Downing Street," the prime minister's headquarters.

What the Americans have done, Macmillan said, is to "stop juggling money and look at the realities of creation of wealth." The United States, he said, has regained its "natural buoyancy" by developing and exploiting new technologies on an immense scale. "We are told today they have too much debt. But how can you have production without borrowing the money to produce first?"

Some of the leading and most able British industrialists have grasped this, Macmillan said, "but most are somehow largely out of it." It is not the old industries that will produce new jobs but the new ones, he said. "This is the test hour" for Britain.

This country "must stop all these futile arguments, these purely theoretical, academic, economic arguments . . . and get back to the reality as it is happening in front of our eyes all over the world," he said, adding that there are much more modern forms of industrial production in Taiwan and South Korea than in Britain.

As for government claims that British exports are rising, Macmillan said that did not impress him because "of course if you halve the value of your money you expect to be able to export."

The coverage that included Macmillan's performance illustrated some of the issues raised in debates about the wisdom of putting legislatures on television.

When Macmillan finished speaking, many in the audience left and the cameras showed those empty seats as subsequent speakers took the floor. Before he spoke, a group of striking coal miners created a disturbance in the galleries. The television crews did not focus their cameras on the demonstrators, in accord with an agreement not to feature interruptions in the proceedings.

The House of Lords traces its history to the 11th century. The average age of the unelected members of the largely hereditary body is 65, as was evident on television.

Lord Chalfont argues that coverage will inhibit candid debate and change the style of members for the worse. He says the networks' interest in the House of Lords, where the debate is more courteous than in the often raucous Commons, is really just a screen to try to get television into Commons.

Public opinion polls indicate 2-1 support for televising Parliament. Supporters foresee more accountability of lawmakers and renewed public interest.