One year ago, on Oct. 19- what has become known as Black Monday - the stock market crashed on Wall Street, losing 21 percent of its value while the Dow Jones average fell 508 points, the worst single-day drop in history. Yet the impact turned out to be less than everyone feared.

In 1929, when the stock market crashed, the national economy plunged into a the decade-long Great Depression. By contrast, the crash of 1987 did not produce a Depression, or even a recession. Today, the economy is perking along in relatively fine fashion.What does this mean?

Well, for one thing, it may mean that businesses across the country pay less attention to what Wall Street does; they tend to focus more on the realities of their own market instead of the stock market. Wall Street does have a tendency to rise and fall for the strangest reasons, sometimes going up with certain kinds of bad news and down with good news.

Still, there are nervous echoes on Wall Street a year after Black Monday. Small investors have not returned to the market in substantial numbers. Brokerage houses are carrying more cash and investing in short-term government bills.

After the crash last year, a number of bills were quickly introduced in Congress, but most have languished, partly because the economy did not falter as expected, and partly because suggested remedies were opposed by many brokerage firms on Wall Street.

One practice that was heavily criticized was so-called "program trading," the simultaneous purchase and sale of large quantities of stocks and the relatively new stock-index futures, to reap profits from the small differences that often exist between the two types of markets.

Curbs on this practice have not been adopted, but several major brokerage houses have given it up, at least for their own investments.

One of the best ways to avoid irresponsible speculation is an easy one - raise the margin required to purchase stock. A "margin" essentially is a down payment on a stock, allowing traders to wheel and deal in stocks while only putting up part of the actual price. Requiring bigger margins would bring more stability to the market. Unfortunately, recommendations to this effect have not been put into practice.

Basically, there are still reasons to worry about the stock market and the economy generally. One is the trade deficit, where the news for August was bad as the deficit jumped by 30 percent. The second worry, and really the cause of the first, is the federal budget deficit.

Until federal deficit spending is brought under control - and progress made toward reducing the national debt as well - the nation will continue to live on borrowed dollars, and borrowed time.