IN THE LATE 19th century, an infectious disease spread through a segment of British society. It was known as Nile fever, and the symptom was a burning curiosity about the source of the river. In that mid-Victorian mania for exploration, nothing so quickened the imagination as the terra incognita of central Africa and the mystery of the river that bisects it. The gaping spaces on the map of the Dark Continent were to the Victorians what the black holes in space are to scientists today, and in the 1860s the literature of exploration had all the dramatic and speculative appeal of science fiction.

The 19th-century adventurers whose magnetic needle pointed in the direction of the White Nile, the river's parent stream, initially knew little more about the region than did the explorers of antiquity -- Herodotus among them -- but within a span of 20 years, they had unearthed the answer to the immemorial mystery of its source. Their names evoke the romance of the quest: Stanley and Livingstone, Burton and Speke, Baker and Gordon.

Lovers on the Nile, by Richard Hall, a British journalist who spent 10 years in central Africa, brings to life at least one of those outstanding personalities in what Alan Moorehead called "the sunburst of Victorian courage and imagination."

Lovers is the story of Samuel and Florence Baker, the unconventional couple that discovered Lake Albert, one of the sources of the White Nile, and the river's largest waterfall, Murchison Falls.

Their 5-year journey through central Africa has all the elements of an H. Rider Haggard adventure and a Conradian fiction combined: disease and disaster, slavery and savagery, pestilence and poisoned arrows. there is more: a lurid secret in the couple's romance. Sam Baker, the quintessential Victorian, a man of letters and private clubs, met his second wife, Florence, not in some fussy London parlor, but in the slave market of the Turkish Balkans, in a town called Widdin, where he purchased her at an auction.

Florence was a slight girl in her teens with braided blonde hair, a Hungarian refugee who had fled one of the uprisings in the Hapsburg empire by crossing the Danube into Turkey -- there to end up, as did so many displaced Slavs, on the white slave market.

By then, the international hue and cry against slavery had begun, but in the Turkish dominions the trade was still thriving. In Zanzibar it was a way of life, a channel of commerce as well-defined as the silk routes of China, as necessary as the spice trade that gave the island its more beguiling imprimatur. It was only in 1873, under pressure of the international blockade, that the Zanaibar slave market was closed. Meantime, slaves roamed the streets like animals, some cowed and domesticated, some wild and near-rabid from prolonged captivity, the rest up for sale and subject ot scrutiny in the marketplace.

In Khartoum, the seat of Ottoman power on the mainland, slavery was even more entrenched, providing manpower for the military garrison which dwindled by a third each year from disease. The slavers replenished their supplies through nocturnal raids on nearby villages, bringing back their spoils -- mainly women -- like cattle, under a yoke of heavy forked poles and crossbars, or chained to the necks of children.

Baker's own attitude toward slavery was not without contradiction. He came from a family whose money was made in the plantation, and hence slave, economy of the West Indies. Despite his condemnation of slavery, he still held the view -- common enough at the time -- that the mind of the African "does not expand -- it promises fruit, but does not ripen." His second journey up the Nile, as a major-general and pasha of the Ottoman Empire, was undertaken with the notion of stamping our the rampant trade on the Upper Nile, and he returned with assurances of "the downfall of slavery." But in fact he had only deflected the traffic from the river to the desert.

Among the many ironies of his crusade was that in order to reach the source of the slave supply at Gondokoro, he needed the protection of salve traders against local tribes. Livingstone had advocated the opening up of Africa in the name of "commerce and Christianity," but the trails cut by Christians like Baker were followed instead by slavers and Islam.

Finally, Baker's behavior toward Florence, while understandable in the context of the era, was far from irreproachable. He dragged her through hardships unknown to most women, and sang her praises in tender tones; yet seven years passed before he told his family about her, and then it was after a furtive wedding in London.

The most intriguing aspect of this book is, unfortunately, the part most wanting. We learn that Florence was hardy, resourceful, clear-thinking and brave, that she was a golden apparition washing her treses in the Nile. Back in England she effected the transition to high society when her husband was knighted with admirable ease, and in that mean-spirited atmosphere (Queen Victoria refused to receive her at court, having got wind of the years in Africa without benefit of clergy), made a touching effort to win the affection of her step-children. When she had had enough of Africa, she lay down the law about ever returning there again, and Baker duly obeyed.

But her childhood and slavery remain blanks. Throughout her travels with Baker, the procession of slaves was endless, a reminder of a fate narrowly escaped and surely perpetually haunting. Yet she makes only one remark about slave traders: "I really hate the sight of them; whenever I see one of them, they remind me of olden times." The comment rings mechanical, and though it is no measure of authenticity, it is also without insight.

Fortunately, a more complex spirit than the "fearless and feminine" is captured in a couple of poignant scenes: When, after unspeakable trials, Florence and Sam finally reach their source of the Nile, she unties the red, white and green ribbon threaded through her hair and binds it to a bush at water's edge, where it flutters in the lake breeze like a miniature Hungarian flag. And at her secret wedding in the vastness of London, her strong, rifle-toting hand is so unsteady that she cannot sign her new name but must cross it our again and again.

Florence leaves much to be examined and explained, but in almost every other way, lovers will fulfill the appetite of exploration buffs. It reveals more than the surface details of quest and discovery, and uncovers the attitudes behind the endeavors (the petty rivalries and squabbles of the explorers, for example).

Happily, it also leaves in the images we crave: lumbering elephants and hungry crocodiles, waterfront caravans and street scenes that resonate like the names of their cities -- Khartoum, Constantinople, Zanzibar -- all suggesting, as the Nile does, both menace and magic.