Why doors are still open at Bell Labs; Three Degrees Above Zero: Bell Labs in the Information Age, by Jeremy Bernstein. New York: Charles Scribner's Sons. 241 pp. $17.95.

''In writing this book I have deliberately not expressed my own point of view on the wisdom of the divestiture. It may come to be viewed as a total folly that disrupted the world's best telephone system. Or it may turn out to have been exactly the competitive spur needed to prod a whole segment of American technology into a viable competitive mode. I do not know,'' Mr. Bernstein writes. ''What I am certain of is that if the divestiture changes Bell Labs' fundamental character - the special mixture of basic and applied science, of long-term and short-term research, of science and engineering - then the United States will have lost one of its greatest technological assets.''

I read every word of this book, even though I didn't understand some of the technical explanations. You might want to do the same, and here are two reasons.

* The book is important in making a strong case for basic scientific research , not only at Bell Laboratories but at academic and industrial facilities elsewhere. At a time when Bell Labs' parent, the American Telephone & Telegraph Company, is deciding what this premier research and development arm (retained after the well-publicized divestiture) should be doing, Mr. Bernstein's book presents facts that argue persuasively for continuation of an evidently marvelous status quo.

* The book is pleasant to read. Parts of it appeared first as individual articles in The New Yorker magazine. Typical of that periodical's profiles, it blends background fact with foreground interview.

At first, ''Three Degrees Above Zero'' looks suspiciously like a sponsored corporate history, one whose friendly tone is obviously the price of whatever sharing of company secrets took place. But this is not the case here, according to the preface. Mr. Bernstein does have friends at Bell Labs, but if the book is uncritical, it is because he personally believes in both what the facility is doing and the way it is going about it.

Bell Labs, its staff cut from 25,000 to 18,000 by the time of this book's appearance, is apparently a wonderful place to work. Some people are engaged in basic research, others in applied research and development. Open-door policies encourage cross-disciplinary consultation. Chemists talk with mathematicians, who talk with physicists, who talk with psychologists, who talk with computer specialists - with synergystic results.

''One difference between Bell Labs and a university is that here, more or less, everyone comes in every day and the office doors are open,'' says Ronald L. Graham, director of the mathematics and statistics center at Bell Labs. ''There does not seem to be any obvious way of knowing how some development here will impact on something over there. You just hope you have good people who are excited and that they can communicate.''

Among the Bell Labs achievements detailed in Mr. Bernstein's book are the development of the transistor, advances in telephony, and discovery that the average temperature of our universe is three degrees above absolute zero. These are interesting things to know about, but in detailing them the author is also making vivid to all of us the trials and rewards of research and development - and enlisting our support.

One of the stylistic curiosities of New Yorker profiles, including the ones reprinted in this book, is the presentation of long paragraphs of material in quotation marks. Such quotes seem to be the result of assorted answers having been put together in such a way that the questions prompting them are implied, rather than stated. Consequently, New Yorker interviewees are made to sound rather uniformly self-centered, and sometimes a bit disjointed in their conversation. Even so, the long quotes work well in a book format.

The implication of Bernstein's book is that citizens like you and me have a stake in all this, whether we know it or not. Just as the first Soviet satellite jolted average Americans into an awareness of space technology and raised questions of whether schools were adequately training enough scientists to enable the US to compete in this field, so here, too, Mr. Bernstein sounds an alarm - though an unimpassioned one - for preparedness in the ''information age.''

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