IT IS A CRUISE for reflection. It is a cruise back to beginnings that one way or another touched half of modern America, perhaps more. It is a cruise without gaiety, a cruise that brings forth a torrent of questions. It is a trip that is over in two hours and yet may be one of the most affecting voyages you can take. It is the trip back to Ellis Island.

Anchored in upper New York bay, just off the New Jersey coast, Ellis Island, named after a farmer who lived there in Revolutionary times, became renowned in th ghettos and peat bogs of Europe. Through the processing center that was opened on this 25-acre atoll, at the very gateway to freedom, passed millions of immigrants, some to find success and even fame in the New World; some to be deported to the dankness from which they had come.

Trim Circle Line boats leave several times a day from Battery Park for the short sail past the Statue of Liberty to Ellis Island (the cruises will end in about a week, but will be resumed next spring). National Park Service guides wait at the gangway to conduct guided tours through the decrepit halls where th paint is peeling, the roof rotted and only the memories and the overgrown grass are green.

The states were responsible for their own immigration until 1890. New York penned its immigrants in Castle Garden, a onetime fort at the tip of Manhattan. The deplorable conditions bordered on scandal.

Washington assumed reponsibility and opened Ellis Island in 1892, a facility designed to process 500,000 new Americans a year. In one 24-hour period in 1902, the crush of arrivals reached 12,000. During the peak year of 1907, 750,000 immigrants went through the inquisition that was Ellis Island.

Today young Park Service guides walk the visitors into the Special Inquiry Room, once a hall of legal questioning. The arrivals were submitted to a barrage of questions. Where did you get your money? (It was suspected that some countries had emptied their prisons and paid the passage of convicts they wanted to unload.) Do you have a job waiting? (A "yes" answer could bring difficulties, for unions were objecting to sharpers who went abroad to recruit cheap labor that would throw native Americans out of work.)

Young, single women, arriving alone, encountered real problems. Women didn't have careers then, and prostitution was a fear. If a girl was sponsored by an aunt or uncle, she would be release only to the aunt. The "uncle" might be a free-lance white-slaver. If a girl was affianced, she was required to marry before leaving Ellis.

Most dreaded of all were the medical examinations. Inspectors lined the way from the ferry landing, up the flight of stairs, into the great hall. Were some who breathed heavily at the exertion?

In his recent book "World of Our Fathers," Irving Howe vividly sketches the scene. A word or two to children to see if they were deaf or mute. A search of the eyes of trachoma. A careful watch for leprosy. A proble into private places for traces of veneral disease. A close look at the scalp for Fauvus, a contagious fungus.

Chalk marks were put on the coats of those to be detained: "E" for eye problems, "H" for hernia, "S" for special inquiry. Of those arrived, 80 percent passed through in three or four hours. Twenty per cent were detained, often in crowded conditions on the island itself. Two percent were rejected. Many of those turned away were children.

A child to be deported had to be sent back with an adult. But one 10 or older could - and often was - sent back alone. The responsibility of the steamship company was to return an immigrant to the port from which the ship had sailed. But the hopeful immigrant had often passed through several European countries enroute to the port of embarkation. Children and elderly people sent back in this fashion were rarely heard from again.

But for those who were accepted there was a new life, and often a new name. The inspectors, tired at day's end, had no patience to deal with complicated European names. Brothers sometimes sailed away from the island with two different last names. An immigrant named McKinley who arrived during the terms of that President was told he couldn't have the same name as the chief executive.

A recent visitor to Ellis told how his forbears boarded the ferry from Ellis bearing the name Gutterman, which was not the name with which they had arrived. During the last stages of processing the immigrant who was to become Gutterman couldn't understand the inspector, so a friendly arrival assured the official, "He's a good-a man, a good-a man." And Gutterman was born.

By no means did all the immigrants who cleared Ellis cluster in New York, German, Polish, Italian and Irish welfare leagues were set up. Religious agencies established a corps of helpers to find relatives, jobs and, in the case of single woman, sponsorship.

So many money changers were cheating the immigrants that American Express was asked to open a bank with a quoted rate of exchange for foreign currencies. Railroad set up ticket windows, and at the peak of immigration were selling 25 railroad tickets a minute. From the misery and the fright of Ellis, 70 per cent of the immigrants went West to provide the muscle for a new America. In a nation searching for roots, this was the real New World beginning.