FSCOTT Fitzgerald in his latest book, "Tender is the Night," has transferred a case study in mental hygiene into a novel of wealthy American thrill-searchers on the Riviera. The performance has been a skillful one.

Rosemary Hoyt, a young motion picture actress, recuperating on the Riviera, meets Richard Diver, his wife Nicole, and their satellites. Among these expatriate Americans "promenading insouciantly upon the national prosperity" are Tommy Barban, an adventurer who has worn the uniform of many nations and is obviously in love with Nicole; Abe Martin, once a musician; the Mckisces who are writing a novel. These swarm about the Divers and gain what stability they have from them, for Nicole is not only beautiful and charming but a successful hostess of her hilltop villa and Dick Diver is handsome and gay, and claims to be the only American man with repose.

In this part a very vivid picture is drawn of the charmingly irresponsible, lavish life of the wealthy on the Riviera but beneath it all Fitzgerald has laid the clue to his story. This is the marriage of the Divers.

The second half of the book takes the reader into the past. Nicole beautiful 16 year old daughter of an American "robber baron" is brought to Europe suffering from schizophrenia, or split personality, resulting from an Flekiva background if is at a Swiss sanivarnon that she meets Dr.Richard Diver, eventually to marry him. The marriage takes place in face of the warning from the other doctors in the sanitarium that it is impossible to be both husband and psychiatrist and do them both well.

The remember of the book describes Dick's effort to refute the doctors' predictions and to retain his self-respect and professional position in the face of Nicole's enormous wealth. Perhaps he might have succeeded if he had not been disillusioned in his love for Rosemary Hoyt, a newcomer to their villa.

Attempting a very difficult task, Fitzgerald has acquitted himself admirably. The contrast of the building up of Nicole after a bad start and the disintegration of Dick whose early start had been phenomenal is skillfully balanced and interestingly developed. "Tender is the Night" is well written, full of action, and true to life