THE Scottish National Portrait Gallery owns an intriguing picture, copied in the eighteenth century from an original which hung for generations at Fordell Castle. To the left sits a crowned personage known by tradition as The Arab Princess. To our right is a dignified black woman, presumably her lady-in-waiting. Behind, a ship sails towards what is clearly the coast of Fife. The image must date from the early years of the seventeenth century, when young John Henderson, who later inherited Fordell, set off for Africa. Just as the name of the artist, bold, but unsophisticated, probably English, just possibly Scots, is unknown, so is the story behind the picture, which was presumably commissioned by Henderson on his way home.

A suspect family tradition has it that, Pocahontas-like, the ''princess'' stepped in to save young Henderson from men who wanted to enslave him. But if true, where? And why did Henderson go to Africa anyway? Fordell lairds enjoyed the profits of a rich coal seam, so he didn't need to seek his fortune. Here Be Mystery - that's the title of the first section of a remarkable exhibition which Dr Polly Rewt has curated for Edinburgh City Art Centre, and it's taken from the despairing caption which an

old cartographer might place over terra incognita, an uncharted section of his map. Contacts between Scots and Africans before the heyday of plantation slavery definitely occurred. But very little is known about them.

And we easily overlook the black presence in Scottish history. Just a few years back, a survey showed that permanent residents of known African and Afro-Caribbean origin could all have been seated in the Edinburgh Playhouse at once. My eye tells me that numbers have grown markedly since, but black men and women are still vastly outnumbered by people of Chinese stock or Indian-sub-continental origin.

In England the arrival in 1948 of a shipload of nearly 500 Jamaicans recruited to serve a British economy desperate, in days of full employment, for extra labour, marked the beginning of a large-scale immigration. But there has only been one such mass entry into Scotland, and it had no sequel. From 1941 to 1944, a ''British Honduran Forestry Unit'' worked in Scotland. It's members, about 1000, recruited from what is now Belize, came from a densely forested tropical environment to work in the chilly Scottish hills. They were warmly received by the locals and many married Scottish women. Some chose to stay here.

By chance I got to know the Scots-born son of a Belizean, working as a labourer in Edinburgh. His wife, too, was Scottish, and their children will ''pass for white''. Intermarriage, more than racism, explains why so few of us are aware of the black heredity of many Scots. Most black people in Scotland have carried, or adopted, European names. The pigmentation fades with each generation.

Which of us can be sure that we don't have black ancestry? There have been black genes in Scotland for almost two millennia. There were African soldiers serving Rome on the Antonine Wall, and black men rowing in Viking longships. James IV employed black entertainers in his court. Black sailors worked ships in and out of the Clyde in the days when slave-grown Virginia tobacco was making Glasgow rich, and Glasgow must have acquired, like other British seaports, a permanent black population, though local historians have ignored it.

Men successful in the New World brought blacks back with them as servants. Some planters sent their illegitimate by-blows in the West Indies back to Scotland for education. The black Scot who rose highest, Sir James Douglas, was one such, the son of a Scot by a Guyanese creole woman. Through service alongside many fellow-Scots with the Hudson's Bay Company, he rose to be the first Governor of British Columbia, around the time that Mary Seacole, from Jamaica, daughter of a Scottish soldier named Grant, made herself more popular than Florence Nightingale, tending the needs of British soldiers in the Crimean War.

Once such researchers as Dr Rewt draw a few curtains, black people stand in the full daylight of Scottish history. James VI's bride Anne brought a favoured black retainer with her from Denmark. One of the people arrested for wearing ''Highland garb'' after that was proscribed following Culloden was Oronoce, a black servant of the Laird of Appin. A black man appears in a picture of Walter Scott entertaining visitors at Abbotsford, and James Hogg's black servant danced at his wedding.

The black novelist Caryl Phillips, who has lived in Scotland and takes a strong interest in the current Roots exhibition, has observed that ''Modern racism is the attempt to justify an economic act: slavery''.

Before the era of the New World sugar and tobacco plantations, it seems that blacks were viewed here as human individuals - exotic, but not unequal. Then, after the Act of Union ''mystery'' was dispelled.

Scots swarmed into the West Indies, dominating whole islands, including the most important, Jamaica. The profits were vast. It is easy enough to document, and explain, the activities of Scottish planters and merchants who exploited slave labour. Slaves brought here were sometimes offered for sale even after 1778 when Joseph Knight, introduced to Scotland by his master Wedderburn, won a judgment from the Court of Session that the latter's claim to own him was ''unjust'' and that slavery was against Scottish legal principles.

Perversely, the long struggle to abolish slavery in the British Empire waged by white liberals and evangelicals and triumphantly concluded in 1833 when a Scottish poet, Thomas Pringle, was secretary of the Anti-Slavery Society, inculcated the idea that black people were mostly childlike victims in need of rescue. Victorian Scots were divided, in general, between those who continued to pity black people and support missions to Africa, and those who thought of blacks as naturally lustful, lazy, and inferior.

But David Livingstone and Mary Slessor knew first-hand from travel in Africa that they were, simply, people. Their status as Scottish missionary heroes didn't prevent the race riot of 1919 which disfigured Glasgow in the heyday of John Maclean and the ''Red Clyde''.

Happily, some black people won respect here. Roots displays individuals such as Thomas Jenkins, a chief's son sent to Britain for education who was for a while a learned dominie in the Borders, or John Edmonstone, a slave brought from Demerara by his Scottish master, who worked as a

taxidermist in Edinburgh

and taught his skills to a

young university student called Charles Darwin, who thought him

''a pleasant and intelligent man.''

Edward Tull, grandson of slaves in Barbados, was adopted from a London orphanage by a Glasgow dentist, trained in his adoptive father's profession, and inherited his successful city-centre practice. His brother Walter, a professional footballer in England, signed for Rangers a few months before he died in France in 1918 - the only black man to be commissioned in the British Army in either world war (As our photo shows, the brothers couldn't have ''passed for white''. Nor could their

sister, whose role was to look after Mrs Warnock, the widowed matriarch.)

It seems that Walter's status as a sporting hero made authorities who were under instructions not to let black soldiers lead whites temporarily colour blind.

Two extended family sagas are featured in Roots. Tiyo Soga, born around 1829, was educated by missionaries in South Africa, then studied in Edinburgh and Glasgow. He was the first African to be ordained as a United Presbyterian minister. He married a Glasgow woman, Janet Burnside, and took her back with him to his homeland, where he achieved lasting reputation by his translation of hymns and the Bible into Xhosa. Their numerous children were educated in Scotland. Dollar Academy became the family's preferred school. Today, great-grandchildren of Tiyo and Janet live in both Scotland and South Africa, and Hector Soga teaches at Dollar Academy.

Edward Awunor-Renner was the first African to hold the post of Director of Medical Services in Sierra Leone. He was one of many distinguished African doctors to graduate in Edinburgh, and there he met his wife, Carmela, a Guyanese resident of the city. She became a noted member

of the International Alliance

of Women and corresponded

with Eleanor Roosevelt. Their granddaughter Susie Goligher went to Cheltenham Ladies College and trained as an art historian. She was drawn into the remarkable story of her heritage only in her twenties and this changed her life. She became a museum worker and is now a devout believer in the importance of studying family history. Dr Rewt hopes

that her exhibition will encourage other families to

go public with archives like Susie Goligher's.

Roots establishes several proud precedents for the current generation of outstanding Afro-Scots which includes the singer and actress Suzanne Bonnar, the poet Jackie Kay, the artist Maude Sulter, and the footballer Kevin Harper. But there are many more still to be discovered. Dr Rewt knows this because once she started looking, she found surprises at such a rate that her exhibition is overflowing.

One story which came her way was this: in central Edinburgh, before the last war, parents despaired of a child's life. None of the medicines prescribed are working. In despair they turn to an African GP practising locally. He came in, swept the medicines off the mantelpiece, and suggests orange juice. The girl lived. She is still deeply grateful.

On the other hand, did you know that in the eighteenth-century a Perthshire lass was kidnapped by Barbary pirates, and became Empress of Morocco? Believe me, this is true . . .

n Roots: The African Inheritance in Scotland, City Art Centre, Edinburgh, May 24 to July 12. Touring Scotland next year.