HOW do we measure greatness in a player? And how can that greatness be converted into a currency that is accepted through the passing of years?

The championing of such as Jimmy Johnstone or Jim Baxter can be bolstered by simply inserting a video into the machine and thus converting the callow sceptic. There is a difficulty, however, when pressing the case of greatness of those such as Patsy Gallacher who have left their marks on the football record books but who have never been captured on film.

In this respect, and in so many others, Gallacher was the original will o' the wisp. There is no grainy newsreel to hint at his greatness. There are few, too, who saw the great man in action and live to tell the tale. Gallacher remains as elusive to the present day fanatic as he was to the toiling defenders who sought to contain him in the early twentieth century.

David W Potter's task, then, is a formidable one. In The Mighty Atom (Parrs Wood Press, #8.95) he seeks to capture the essence of greatness and make it a solid reality for the reader.

He brings to his work an enthusiasm in style and an assiduity in research that almost breathes life into the great Patsy. The dusty record books contain contemporaneous accounts of a remarkable career but Potter has tried to put some flesh on the bones of the waif-like Gallacher who stood just 5ft 7ins and weighed less than 10st at the start of his career.

There is much of Gallacher's illustrious playing career for Celtic and then Falkirk but Potter lifts this from the realms of prosaic fact by trying to place the Atom in a historical context.

Gallacher was born in a poor house to illiterate parents and immigrated from Donegal to John Knox Street, Clydebank, at the turn of the twentieth century. Potter does not skirt the sectarian and political issues of the era but he is at his most adept when detailing the career of a man who many believe was the greatest Celt ever. Two of his strongest advocates for that title were Willie Maley, the long-serving Celtic manager, and Allan Morton, who as a Rangers player witnessed much of the mayhem that Gallacher was able to create with a slip of the shoulders or a well-crafted pass.

Gallacher joined Celtic in 1912 and immediately won a Scottish Cup medal. His career ended at Falkirk when he was approaching 40.

This span of 20 years covered triumph on the park and tragedy off it. His wife was only 35 when she died after giving Gallacher a sixth child. He accepted his responsibilities and carried on with the same quiet fortitude that enabled him to survive on the park in an era when a ball player was regarded as a legitimate target.

Potter's research covers all the public glory and some of the private heartbreak. Due tribute is paid to Gallacher's goal in the 1925 Scottish Cup when the great one scored in a crowded penalty area by the simple expedient of lodging the ball between two feet and somersaulting into the net. Recognition is made, too, of Gallacher's uncanny ability and his quiet resolution in inspiring the lesser souls that surrounded him.

The straitened conditions for footballers in that bygone era are also beautifully invoked. Gallacher once missed almost an entire season because he played in borrowed boots that were too small, thus poisoning his toes. He also missed games after dropping a tool on his foot while working in his day job as a shipwright on the Clyde.

But Gallacher's genius can not be confirmed through the record books. Other Scottish players have played more, scored more, won more.

Potter's trump card is his questioning of the witnesses. Gallacher was the founder of a footballing dynasty. His sons played professionally and his grandson is Scottish internationalist Kevin Gallacher. The Divers' family tree has also its roots in Gallacher soil. Potter's antecedents are more humble in footballing terms. His father was merely a fan but his testimony, allied with so many others of that era, has the simple power to convince.

When arguments raged about the relative merits of Johnstone, Pele, or Puskas, Mr Potter Sr delivered a summing up which brooked no contrary verdict over who was the greatest of them all: ''Look, I've seen Patsy Gallacher.''