A round gold pocket watch the size of a kumquat tells the story: On it is inscribed a large, unblinking eye, the family crest of the Pinkertons. ''We never sleep'' was the motto of the Pinkerton National Detective Agency (now Pinkerton's Inc.), whose dramatic first 50 years are brought to life vividly in an exhibit at the National Portrait Gallery here.

Sam Spade, Nero Wolfe, or Ellery Queen couldn't match the real-life thrillers found in the files of the agency that tracked Butch Cassidy and the Sundance kid across half the United States, tangled with Jesse and Frank James, and derailed the train-robbing Reno Gang.

The Pinkertons got their incongruous start in 1850 in -Chicago when a bold Scotsman, who'd gang aft a-gley of the law in his native Glasgow, decided to turn his talent for sleuthing into a lucrative living.

The dour, bearded Allan Pinkerton was a 22-year-old barrelmaker in Glasgow when his radical views on reforming England's Parliament got him into trouble with the law. As a fiery protest leader in the Chartist movement, he was the object of a warrant issued with a reward for his arrest. So Pinkerton skipped Scotland with his bride and fled to Dundee, Ill., to make a new life in America. He became a successful barrel manufacturer and had eight men employed in his cooperage when he found the first clue as to what he would be for the rest of his life. Out searching for barrel staves one day in 1847 on a deserted Illinois island in a river closeby, he found a pile of smoldering ashes that looked suspicious. He crept back a few nights later in the dark, screened by shore grass, and discovered a gang of coin counterfeiters around a campfire.

Their capture by the local sheriff, as well as Pinkerton's next success in unmasking a bank note counterfeiter named John Craig, established his reputation. From then on it was only a couple of years before Allan Pinkerton became -Chicago's first full-time detective. By 1850 he was head of his own detective agency.

The story of what happened next is on the Victorian taupe-and-brown walls of the ''We Never Sleep'' exhibit at the -National Portrait Gallery. There are enough plots in the first 50 years of Pinkerton history to make a movie mogul weep with greed.

And indeed, some of the Pinkerton exploits have already turned up in films. Who can forget the flinty-eyed Pinkerton man never blinking in his relentless gallop across the plains after Robert Redford and Paul Newman in ''Butch Cassidy and the Sundance Kid''?

Director Sam Peckinpah chronicled the violence of ''The Wild Bunch,'' the larger gang that Butch and Sundance once belonged to, in his movie by that name. The James brothers have turned up as often as sagebrush in Hollywood westerns, most recently in ''The Long Riders.'' And Richard Harris starred in the film of ''The Molly Maguires,'' the story of a secret society of Irish immigrant mine workers in Pennsylvania.

In 1874 the Pinkertons were called in to investigate whether the Molly Maguires had destroyed Philadelphia and Reading Railroad cars as part of a strike tactic against mine owners. The agency sent in a gumshoe from Ulster, a convivial red-headed Irishman named James McKenna, who penetrated the clandestine society and became a trusted member. His discovery that the society was for the most part a group of hoods bent on carrying out personal vendettas against -rival gangs, terrorizing and even murdering their enemies, resulted in the arrest and execution of 20 murderers and the end of a long crime wave. It also brought Allan Pinkerton's agency the sort of national fame money couldn't buy.

Unfortunately, a few years later, after Allan Pinkerton's death, the agency was involved in an affair that was to -become as infamous as the Molly Maguire episode was famous. In what is now called the Homestead riot, 315 Pinkertons were sent to put down a union strike against the Carnegie Steel Company's Homestead, Pa., mill in 1892. Hundreds were left wounded and 16 dead in a daylong battle that began when the Pinkertons, hired because of rumors of a union -seizure of the mill, began firing into a crowd of strikers.

''As for the Pinkertons, their involvement in this labor dispute, as in others, left them with a tarnished name among the working class, a name that the agency attempted to rectify through its success in tracking bank robbers, forgers, and ringers,'' point out Frederick Voss and James Barber, authors of the fascinating catalog accompanying the Portrait Gallery's exhibit.

The Scottish radical who founded Pinkerton's showed his allegiance to his new country by offering his agency's expertise to President Lincoln in what amounted to the first Secret Service. In fact when Lincoln was first elected, Pinkerton was responsible for smuggling the new president into Washington through Baltimore, where he had to make a speech despite threats of an assassination attempt. ''Secrecy is the great lever I propose,'' he wrote to Lincoln. Pinkerton saw to it that the telegraph lines were cut, the railroad tracks guarded, and messages sent in code, as Lincoln sped through Baltimore on a night train to the capitol. We're told that Lincoln, an open man, later regretted the stealth with which he entered Washington.

As head of the secret service for the Union Army, however, Pinkerton had a less than distinguished career. But he was responsible for one dramatic coup. It involved a certain Confederate Mata Hari, a handsome brunette named Rose O'Neal Greenhow, who in 1861 was described as ''the most enchanting woman in Washington.'' But while Rose Greenhow laughed and flirted behind her lace fan, she was busy collecting Union military secrets from the senators, representatives, and aides who admired her, then passing them along to the rebels. Apparently one of her reliable sources was a married admirer, Massachusetts Sen. Henry Wilson, chairman of the Military Affairs Committee; a love letter to her signed ''H'' is part of the exhibit.

Also in the exhibit is a delightful wood engraving of a barefoot Allan Pinkerton tracking a Confederate spy in the rain from Rose's house on 16th Street. Pinkerton had posted a watch outside Rose's house in a downpour and decided to get a better look as a federal officer passed military secrets to her in an upstairs parlor. To peer in, Pinkerton took off his boots and stood with one foot on either shoulder of two detectives. The Union man suddenly bolted from the house. Pinkerton ran barefoot after the traitor but was stopped by armed Union soldiers, who arrested him presumably for his dangerous-looking feet. Pinkerton, secretive as ever, decided not to spill the beans until he had more evidence.

He finally got it at the end of August, when Mrs. Greenhow was placed under house arrest and her small red diary, containing lists of couriers, fellow agents, weapons reports, and troop movements, convicted her. There is a poignant photograph of the lady as jailbird in the exhibit. Taken from a negative by Mathew Brady, it shows Rose Greenhow sitting against the barred, brick wall of the old Capitol Prison. She is wearing a black chiffon gown with ruffles, black kid gloves, and a mass of coquettish curls. Her young daughter is leaning at her side.

It was one of the compassionate paradoxes of Pinkerton's makeup that although he was in the law-and-order business, he made one crucial exception. From the time he landed in the US, Pinkerton had been ardently antislavery. Before the Civil War broke out he had repeatedly and secretly broken the law by helping runaway slaves find freedom. When his antislavery feelings catapulted him into volunteering his agency on the Union side, he took his two sons, William and Robert, with him. After his death they continued to expand the national sleuthing of ''the Pinks,'' as they came to be called, capturing some of the most famous names in crime, from Adam Worth, the ''Napoleon'' of international crime, to Sophie Lyons, ''Queen of the Underworld.

Even some of the less prestigious criminals achieved a sort of fame through being tracked by the Pinks. Part of the exhibit is the brief story of ''Terrible Fred,'' a glum-looking express-train robber named Fred Wittrock who had read too many dime novels. When the Pinkertons caught up with him he was dubbed ''Terrible Fred'' at his own request, and the name stuck.

Pinkertons specialized in not only getting their man (or woman) but also noting the criminal's giveaway characteristics or distinguishing marks with a novelist's eye. We learn that Dink Wilson, a notorious train robber, had a bad case of blackheads and a swinging gait and that Underworld Queen Sophie Lyons, a pickpocket at the age of 6 who later married ''a bank sneak,'' had ''an underlip that puckered when she talked . . . and was a confirmed opium fiend,'' according to the Pinkerton file. James Dunlap, the bank robber who made off with over $1 million in 1876 (when it really was $1 million) from a Massachusetts bank, had a first in the Pinkerton's file. As a prisoner with a fan club of adoring women, he made money in jail by sending pictures of himself along with locks of his wavy red hair to his admirers for a quarter each.

In this century, Pinkerton's traipses on, perhaps a little less colorfully (since only 10 percent of its business is investigative now), but certainly much more lucratively. In 1980 Pinkerton's Inc., had net revenues of $285 million. The firm has 115 offices in the United States and Canada; an additional one opened this year in London. According to George O'Neill, director of personnel for Pinkerton's Inc., 90 percent of the firm's revenue comes from security work - uniformed guard work - ''but we still do $15 million worth of investigative work.'' Not bad, for a firm that started with a curious Scottish barrelmaker noticing a few ashes around a campfire.

The Pinkerton exhibit, ''We Never Sleep,'' can be seen at the National Portrait Gallery through Jan. 3.

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