Twenty years separate the Hollywood F. Scott Fitzgerald described in "The Last Tycoon" from the Hollywood of Jill Robinson's new novel "Perdido."

Of those hectic '30s when studio bosses like Harry Cohn or Sam Goldwyn averaged five pictures a month, Fitzgerald wrote pointedly: "One doesn't mix motives in Hollywood. A mixed motive is conspicuous waste."

The philistinism of those tycoons sometimes made them laughable; but it also left them no choice but to trust their own instincts, make their motives clear, smite the sophisticates with an unnerving candor.

If they did not adapt to the sly, new world their subordinates were hatching through the '30s, they still stood for, as Fitzgerald put it, "escape into a lavish, romantic past that perhaps will not come again in our time."

Jill Robinson's Hollywood of the '50s - she is the daughter of Dore Schary, production chief of M-G-M in the early part of the decade - had lost its candor to McCarthyism, its momentum to television; but it's glamor survived even the distaste for glamor sworn by the young lions of the day, Brando and James Dean and Montgomery Clift.

Because Robinson's book is bound to be compared to Fitzgerald's, and because it is the first "Hollywood" novel to stand the comparison, it is worth emphasizing that the differences cannot be attributed simply to the changes in Hollywood between the advent of talkies and the advent of TV. Rather, they lie in the contrary use to which each author puts the past.

In a milieu in which people become "legends in their own time," the past is not easy to pin down. Beyond a certain point, Fitzgerald did not even try. He invented his own mythology, nobler no doubt than that of the M-G-M publicity staff, but not radically more accurate. The little chivalries and little treacheries of men like Harry Cohn become in the character of Fitzgerald's tycoon chivalry, treachery - moral drama - on a romantic scale.

"Perdido," on the other hand, emphasizes that we only diminish the present by blowing the past out of proportion. Robinson's novel is about an adolescent girl who must face that fact in, of all places to face unadorned fact, Hollywood.

Fifteen-year-old Susanna Howard is the granddaughter of the tycoon who founded Levanin Studios and built Perdido, where Susanna has been raised. Her stepfather now runs the studio; her mother runs the dinner-table, plays an odd sonata, leaves Susanna to the care of servants. We first meet Susanna on a bicycle heading (a la Esther Williams) straight for the pool. She surfaces to find one of her screen idols, Jackson Lane, applauding her "aquacade." To her delight, Jackson encourages her to play hooky and join him for a day on his sailboat. ("Even before I heard about Jackson I wanted to run away to sea in a boat. This is what boys do in movies to grow up.")

Despite her self-chiding for her crush on Jackson and all the other romantic intrigues she pursues through out the novel, Susanna has no foot-hold in the real world to stay her downfall into fantasy. Why her mother and stepfather have not filled Perdido with the warmth of "real families in the movies," she is the last person in Hollywood to know. The search for the truth about her past which her mother has kept hidden from her takes Susanna to the Mid-west, Florida, New York, all across the country.

Her expectations readers cannot help sharing, at the same time that we suffer, knowing that only in a Shirley Temple movie could dreams like Susanna's come true.

The wonderful thing about Robinson is that she does not leave her poor Susanna to the indifferent symmetry of an unhappy ending.

Her last "reel" takes Susanna to 1960, six years into her marriage to Paul Daroff, the heir to Daroff Studios - and to 400 filmscripts remaining when Daroff Studios, like Levanin Studios, fell to TV people in the '50s.

While there are nights when Susanna thinks of Paul as a Scheherezade, postponing sex until he has finished reading one more of his 400 properties, there are also moments when Susanna understands how intimately he, no less than she, has been damaged by the lies he learned to live with as a child.

Learning to love, Robinson makes beautifully clear, is impossible without an honest scrutiny of one's own past. Susanna, in cameraman lingo, warns us that "memory has a way of shooting through Cheesecloth, diffusing character."

Shooting through cheesecloth, differing character, may serve the purposes of an artist like Fitzgerald who can keep his nostalgia in the rein of morality. But for lay romantics like most of us - and like Susanna - counterfeiting the past can only distort our efforts to make sense of the present. Perdido is a realist's rebuttal of "The Last Tycoon." And in its own way, it's just as good.