"YOU SHOULD NOT undertake things which you cannot accomplish. You are always too adventurous." So Mrs. Disraeli warned Prince Louis Napoleon. It was 1838, and the prince was in exile in London. Rowing the Disraelis on the Thames, he had forced the boat on to a mud bank. Mary-Anne Disraeli was renowned for her lack of tact; time she showed a lack of perception. Louis Napoleon was one of the great adventurers of modern history. Ten years after he ran her aground on a mud bank at Fullham, he was elected president of the French Republic; four years after that, he became the emperor of the French. Lord Malmesbury, who knew him well, declared that the most remarkable feature of his character was his obstinacy of intention. "All projects once formed and matured in his head," wrote Malmesbury, "remain there, perfectly uncommunicated in detail, but their pratical attempts at fulfilment will be a mere question of time. To be Emperor has been his marotte since he was twenty years old."

As Jasper Ridley shows in this dual biography, Napoleon III and Eugenie, he was driven not only by his merotte , or fixed idea, but by his superstitous belief in his destiny; he had courage, and he possessed an intellect of no mean order. He was ardent patriot and a philanthropist; he was concerned with the glory of France and with the welfare of its citizens. He sought for glory in the France-Prussian War. He also showed a paternalism which owed much to socialist theories. He not only presided over Haussmann's transformation of Paris; he made plans to rehouse the lower classes, and to provide relief at times of distress. He set upa system of free distribution of food in French cities whenever there was a shortage of food in the winter months. Part of the cost was paid from his own personal allowance. In 1853 he ordered the director of public assistance to organize a system by which doctors would visit the poor in their homes and give them free medical treatment there, to relieve the overcrowding in the hospitals.

Only the most violent republicans would have denied his desire to go good. Dr. Barthez, who was to be the prince imperial's physician, had occasion to study and emperor at close quarters, and he was, above all, impressed by his benevolence. "It is curious to see how stronly this man has the instinct of practical well-doing . . . The more I see this man, the more closely I study him and the more attached I grow. I have looked for his worse side in everyday life and I have not found it . . . With the emperor, his only fault, if it is one, is that his kindness errs to the verge of weakness. Of course it is of the private man I speak." Emile de Girardin, the most influential journalist of the time, described him as Napoleon the Well-Meaning, a title subtly blending compliment with criticism. Arsene Houssaye, at the end of the century, declared that the emperor had been lost by his love of concilliation; but it is probably true that, if his reign had ended successfully, his personal qualities would have exalted him to the skies.

Much has been written about Louis Napoleon -- his well-publicized love-affairs, garnish society life of the Second Empire, the disasters of 1870. Yet Napoleon le Petit remains a much larger figure than Victor Hugo, or his other critics, have allowed.

His latest biographer, Jasper Ridley, presents him with affection and due respect in this meticulous and massive book. He presents the Empress Eugenie with sympathy, if without the fervent admiration shown by the late Harold Kurtz in his excellent biography in 1964. Eugenie remains exceedingly complex: a feckless, intensely romantic girl, an ambitious and perhaps disasstrous empress, and, for her last half-century, an almost epic figure, living through her English exile with a quite exemplary fortitude. The death of her husband was not perhaps unexpected (though Ridley makes some interesting comments on its cause). The death of her only son, the prince imperial, was shattering. He was killed, at age 23, on active service with the British Army in Zululand. Five days after she heard the news, she wrote to her mother: "Today I have the courage to tell you that I am still alive, because pain does not kill."

She lived for another 41 years. She owed much to her moral and physical vitality, and much to the practial affection of Queen Victoria. The Bonaparte future had died with her son, but she paid occasional visits to Paris, and built herself a villa at Cap Martin. She cruised in her yacht, the Thistle, to Norway and Ireland and Constantinople. She entertained European royalty. She continued to make friends: among them Kenneth Clark (then a child of 7), and Jean Cocteau, who listened to her conversation, spellbound, fearing that if he moved he might "make the cock crow and send the ghosts away."

All this and more Ridley records with scholarly precision. His research has been extensive; but he has said too little, to my mind, about the emperor's broken engagement to his cousin, Princess Mathilde (was this the mariage mangue of the age?). He has not, perhaps, done justice to Prince Napoleon, or said as much as he might have done about the prince imperial and about the vicissitudes of Bonapartist politics. He does not give an adequate account of the social and cultural background of the Second Empire. As a dual portrait, however, Napoleon III said Eugenie is a solid and commendable work. Even the specialist should find something new in its 700 closely printed pages.