RATNER'S - 138 Delancey St., New York, N.Y. (212) 677-5588. Open: Sunday through Thursday 6 a.m. to midnight, Friday 6 a.m. to 3 p.m., Saturday sundown to 2 a.m. All major credit cards. Reservations accepted. Separate non-smoking section. Prices: appetizers $6.50 to $9.50, entrees $4.25 to $17.95. Dinner with drinks, tax and tip about $15 to $25 per person.

SIGHTSEEING IN ANOTHER CITY doesn't end at sundown. Dinner offers one of the best chances to get to know a new place, to watch its people, to learn its legends.

In New York, a good place to start is Ratner's, the legendary dairy restaurant on Delancey Street, the heart of the Lower East Side. Before the rubber chicken gained national prominence, before Jewish-immigrant movies were nominated for awards, Ratner's was a necessary stop on any Manhattan campaign trail. Before Moscow waiters totally squashed any competition in the surly server awards, Ratner's waiters were famous for scowls that theoretically hid hearts of gold. Before cholesterol was discovered, Ratner's was the epicenter of sour cream cookery -- and it hasn't yielded its ground.

Delancey Street, in the shadow of the Williamsburg Bridge, is the second location for this 85-year-old restaurant, but Ratner's has been on Delancey long enough to watch the neighborhood turn more Hispanic than Jewish. It's still glatt kosher -- meaning the ultimate in kosher -- but it is no longer priced for penniless immigrants. It has also modernized enough to include lasagna and frozen yogurt on the menu (plus chow mein and chop suey, relics of an earlier modernization).

The most significant change in the Ratner's of today is that its waiters are NICE. I wondered whether I was in the right place when ours cheerfully moved us to a new table after more friends showed up. Our waiter didn't snarl when we ordered our pirogi half-potato and half-kasha. And when he kept offering us more water, I began to wonder whether I was in New York at all. He brought extra plates for sharing, and even looked grateful for the tip. In fact, the only scornful look our waiter affected was when we branded ourselves as tourists by trying to order an egg cream.

What hasn't changed about Ratner's is that people still complain that the onion rolls aren't nearly as good as they used to be. That lament was probably invented the first week Ratner's opened.

As for the dining room itself, it obviously has remained intact for decades -- unless a factory specializing in antique Naugahyde lurks somewhere. Surely Ratner's particular shade of taupe stylized wood grain was long ago banished from the Formica factories. And nowhere outside of an MGM musical set would anyone still design such a coral-edged, curved, dropped ceiling with recessed lighting and funky brass-tone chandeliers.

The cooking too is authentic first-generation Jewish. The bakery cases in front set the tone with their thick-crusted pies and shiny coffeecakes. This is food valued by the pound.

Ratner's menu is a poem to the potato. Potato pirogi, potato blintzes, potato pancakes, potato soup. The potato pancakes, deep-fried and crunchy, are grand, but the soup is the star among the potato dishes. But then Ratner's soups are so successful that they have been marketed nationwide in freezer compartments. (The borscht is also excellent, as well as the most beautiful pink in the world.)

What roots Ratner's in tradition is lumps and chunks. The potato soup, faintly rust-colored and slightly thickened, has potatoes in chunks, even halves. It's nothing like that bland, smooth pap that other restaurants dignify with the name vichyssoise. Even the gefilte fish is a big irregular lump the size of a small loaf of bread: homey, lovable.

Anything that's not made from potatoes is likely to involve dough. Just beware of those dishes that are both potato and dough -- the potato pirogi are best appreciated on a December day in Siberia. In fact, even the cheese-filled kreplach are dauntingly doughy.

If you're looking for something both heavy and wonderful, look on the menu under the "Mushrooms and Vegetables" category (what other restaurant has such a category?). Once you've realized that there are three different versions of baked brown kasha, you know you are onto something. I love it with varnishkes -- bow-tie noodles -- and creamed mushrooms, which means the kasha is drenched with a hauntingly delicious cream gravy with chunks of mushrooms.

The fables about Ratner's usually revolve around blintzes. If a politician is seen lunching at Ratner's, the event shows up in the press as "eating blintzes at Ratner's." And the long lives of the waiters are attributed to their steady diet of blintzes and sour cream. What's more, the blintzes come in seven varieties, ranging from potato, of course, to blueberry. The blintz wrappers are impeccable, eggy and thin, fried so that the corners are crunchy. And the cheese filling, though too sweet for my palate's tradition, has an old-fashioned crumble to its texture.

You'd expect great smoked fish at such a legendary restaurant, and the whitefish and sable outshine Washington's best. But they don't compete with New York's or Miami's best. And while the scrambled eggs with nova are soft and comforting, with the smoked salmon just warmed so it hasn't turned tough and salty, they are not New York's finest.

To verify that, I went over to the Second Avenue Deli, which is the remaining great kosher meat deli of Lower Manhattan. On its sidewalk are markers with the names of the great actors and musicians of the Yiddish theaters that once lined Second Avenue. Here the Naugahyde is green rather than burnt siena and the menu runs to cholent and chicken in the pot. And the lox omelet is of course made with oil rather than butter. But it is sensational -- soft and creamy scrambled eggs with large silky flakes of smoked salmon. And when the waiter served it to my son he announced, "Here you go, Doctor . . . They made this like it was made for a doctor." Now that's a Jewish waiter.