''Under the Rose'' is an early version of Chapter Three of ''V,'' in which Herbert Stencil plunges into a past Mr. Pynchon constructed out of Baedeker's ''Egypt'' and English spy novels. The year is 1898, the time of the Fashoda incident, which brought France and England to the brink of war and served as a premonition of the violent new century to come. In ''V'' the chapter is important because it introduces the lady V herself, Victoria Wren, of Horkshire, whose ''natural habitat'' is said to be ''the state of siege,'' and who comes to be associated with all the horrors and cataclysms of modernity.

As a separate story in this collection, with certain passages more fully developed (or not yet fully trimmed), ''Under the Rose'' tells the story of ''V'' in miniature but without the baleful woman as a focus. It recounts the subtle transition from a world governed by rules and persons to one based on pathology and anonymous ideas. The English spy Porpentine and his German counterpart Moldweorp are enemies but ''cut from the same pattern: comrade Machiavellians, still playing the games of Renaissance Italian politics in a world that had outgrown them.'' They operate now ''in no conceivable Europe but rather in a zone forsaken by God,'' and the place begins to look like the location of ''Gravity's Rainbow.''

''It was no longer single combat. Had it ever been?'' The last, sudden question expresses a characteristic, brilliant Pynchon worry. The world has changed, ours is the century of horrors, the loss of our humanity is our favorite human cliche. But has the world changed in that sense, and if it hasn't, what are all those fictions of change about? Did we have all that much humanity to lose? ''Suppose,'' Herbert Stencil's father says in ''V,'' ''sometime between 1859 and 1919, the world contracted a disease which no one ever took the trouble to diagnose because the symptoms were too subtle - blending in with the events of history, no different one by one but altogether - fatal.'' Can there have been such a disease? If not, did nothing happen between 1859 and 1919 except the passage of time and the brutalities that never go away? Porpentine is killed among the pyramids, but his colleague survives, and ''sixteen years later, of course, he was in Sarajevo,'' failing to prevent another, more famous assassination.

Grover Snodd, in ''The Secret Integration,'' is a ''boy genius with flaws'' - too dumb, his friend Tim thinks, to cover up how smart he is. He and his pals lived in Mingeborough, in the Berkshires, and are carefully preparing a children's revolt on the model of the slaves' uprising in the movie ''Spartacus,'' which they have just seen. As the story proceeds, it becomes clear that the revolt won't take place, that the children's ''insecurity and discontent'' are not all they might be, and that mums and dads are more needed than resented: ''There was a point at which the reflex to their covering warmth, protection effectiveness against bad dreams and simple loneliness took over and made worthwhile anger with them impossible.'' A familiar enough perception, and it is set against the children's discovery of adult isolation. Tim is asked to make a phone call to Los Angeles and thinks, like Jack Kerouac, of America spread out in the night from coast to coast, ''and how hard it would be, how hopeless, to really find a person you needed suddenly, unless you lived all your life in a house like he did, with a mother and father.'' But the man Tim is telephoning is a black musician, an alcoholic far from any home he ever had, and through him Tim and Grover and their friends learn not only about loneliness but about color, and how people feel about it in the cosy white town. The one black family in the neighborhood is repeatedly, violently abused, and a heap of garbage is dumped on its lawn - sad, compromising muck in which the boys recognize their own parents' refuse. It is at this point that the boys' black friend Carl, full member of the Spartacus team, is revealed to be a phantom, a pal they have constructed in memory of the black musician they could do nothing for. ''He was what grownups, if they'd known, would have called an 'imaginary playmate.' His words were the kids' own words; his gestures too, the faces he made, the times he had to cry, the way he shot baskets; all given by them an amplification or grace they expected to grow into presently.''

The revelation of the black boy's status is awkward, but there is really nothing fantastic about the story. It is the most solidly specified of Mr. Pynchon's works before ''Gravity's Rainbow,'' and has dense and affectionate evocations of old houses, hotel rooms, sounds in the night, snatches of children's talk and games. In this tangible context, in the irrefutable reality of this familiar territory, the black friend has to be a phantom. If he were real, no one would let him in. The only integration possible is a secret one, and even that must end, because the children ultimately need their parents, as their parents need their prejudices. Entropy, the defensive torpor of Lardass Levine, the feelingless low- lands of Dennis Flange's dream, Porpentine's projected kingdom of death, have stealthily covered the earth, only their name now is maturity, the life children learn to live.

THOMAS PYNCHON was a cult figure in the mid-60's. Copies of ''V'' were passed around and annotated amid the Dylan records and the beginning of the end of the Beatles. He was then taken up in a big way by the academy and must be now among the most written about of contemporary authors. I have the highest opinion of Mr. Pynchon's work myself, but what I miss in the figure he has become for scholarly critics, in the difficult, meditative writer who is thought to put all merely lucid or entertaining practitioners to shame, is the sense of a man in a particular time and place, and of a living author whose faults as a writer are not to be extricated from his great virtues. This is just what ''Slow Learner'' helps to restore.

''What is most appealing about young folks,'' Mr. Pynchon writes at the end of his introduction, ''is the changes, not the still photograph of finished character but the movie, the soul in flux.'' ''Maybe,'' he continues in a slangy voice that bears history along just because it is dated, because you can hear the time in it, ''this small attachment to my past is only another case of what Frank Zappa calls a bunch of old guys sitting around playing rock 'n' roll. But as we all know, rock 'n' roll will never die, and education too, as Henry Adams always sez, keeps going on forever.'' B