WHAT IS it about scientists that compels them to write books, in particular books of essays? Whether it is physicist James Trefil reflecting on the nature of the physical world, neurologist Oliver Sacks relating his bizarre clinical tales or oncologist Lewis Thomas drawing lessons from life at the cellular level, each of these accomplished scientists (and many more) has taken time from his main pursuits in laboratory or clinic to write.

There are of course many motives for writing, and many kinds of scientific writing. The worst of it is self-aggrandizement; the most innocent is the popularizing work of such as Stephen Jay Gould and Carl Sagan, who want to convey to the masses the importance and wonder of science. But the scientific essayists are another breed, less concerned with their own work, or even with explaining science. Rather, they tend to focus on scientific method, on how scientists see and sort through their observations to arrive at something useful. At their most ambitious they are turning to questions that the empirical method cannot answer; they are taking the skills of the scientists, the keen observational and analytical senses, and using them to explore (to essay, if you will) the nature of self and society.

Three new works -- two collections of essays and the third an extended essay -- demonstrate the range of scientific reflection. The most entertaining is physician Gerald Weissmann's They All Laughed at Christopher Columbus: Tales of Medicine and the Art of Discovery (Times Books, $17.95), an examination of (among many other things) the way doctors diagnose. Weissmann, an inflammation specialist at New York City's Bellevue Hospital and author of The Woods Hole Cantata, opens with an essay on Sherlock Holmes ("The Game's Afoot at Bellevue"), whose acclaimed observational and diagnostic skills were modeled on the dazzling clinical abilities of physician Joseph Bell. He then narrates several case studies of his own, including: a case of Munchausen syndrome ("The Baron of Bellevue"), a bizarre psychological disorder characterized by the invention and creation of disease symptoms (in this case, symptoms of an endocrine disorder) where none really exist; a case of Reiter's syndrome (the title essay), which may have been responsible for the arthritis and inflamed eyes that severely debilitated Columbus in his final years; and a probable case of hepatic porphyria ("Marat on Sabbatical") which had the French revolutionary and physician Jean-Paul Marat confined to the bathtub in which he was assassinated.

Weissmann uses his case studies, historical and actual, as starting points for digressions on all sorts of matters. Indeed, there are as many digressions here as there are tales of medicine, including: a critique of sociobiology; a discussion of the state of medical education; a meditation on Austrian medicine in 1938 ("Springtime for Pernkopf"); and a satirical look at the book publishing industry through the eyes of Voltaire. Some may find Weissmann's conceits a bit strained, but his prose is graceful and his cultural range remarkable. Collectively the 23 essays and skits illustrate his recurrent theme: the inexactitude of medicine because of the influence of historical fashions and the limits of human perception.

PERCEPTION and its limits provide the theme for a second volume of essays, psychologist Richard L. Gregory's Odd Perceptions (Methuen, $19.95), a strange but enjoyable look at the ways people use their senses to make sense of the world. Gregory, a professor of neuropsychology at the University of Bristol and editor of the journal Perception, defines his topic very broadly, including, as he says, not only "colors, shapes, sounds, touches, tickles, tastes, smells, and so on for the many other senses -- but also, and most interesting, how the signals from the senses interact with our intelligence." Now that's vast territory -- one might call it human consciousness -- but Gregory explores it gamely. His reach takes in jokes, or how we perceive funniness ("Laughing Matter"); intention, or lack of it, in robots and computers ("Toy Mates"); how conjuring works; and illusion ("The Oddest Perceptions"). He also offers historical reflections on the scientific contributions of Johannes Kepler, Denis Diderot and Francis Bacon, and he even recreates his own personal experiment with chemically induced loss of consciousness ("Journey to Unconsciousness with Ketamine"). Gregory, an expert on artificial intelligence, is interested ultimately in the inadequacy of human thinking, and in these 31 short takes he ably demonstrates the constricted nature of human awareness.

WILLIAM SARGENT, author of The Year of the Crab: Marine Animals in Modern Medicine (Norton, $14.95), is also interested in perception, though his reach is not so lofty. He is interested in how scientists see, in particular how marine biologists see in marine animals the potential for understanding (and vanquishing) human disease. Sargent's historical example is Elie Metchnikoff, the Russian-born biologist who in 1883 was the first to observe that cells fight infection; he observed the immune reaction in a starfish larva, and ever since, Sargent maintains, scientists (and especially the scientists at the Woods Hole marine biology lab) have been using their keen observations of marine animals -- sea urchins, horseshoe crabs, lobsters -- to make a contribution to human welfare.

Sargent comes very close to promising a cure for such disorders as Alzheimer's disease and Lou Gehrig's disease, and in order to do so makes a conceptual leap worthy of Evel Knievel.

Which is too bad, because there is also a good book contained in this slim volume. As Sargent notes in his acknowledgements, he was writing a natural history of horseshoe crabs before his editor convinced him to describe the contribution of marine biology to biomedical research. The editor was wrong. Fortunately, the natural history is still in here (in fact, the book's title is an artifact of that earlier effort); it's a bit fragmented by the rah-rahs that have been interspersed, but it makes delightful reading for a hot summer day. Sargent is clearly enamored of nature and should probably stick to writing about it.

SCIENTISTS, of course, are not the only ones writing about science. Journalists do, too, and when that happens, according to Cornell sociologist Dorothy Nelkin, the results are unhappy. In Selling Science: How the Press Covers Science and Technology (W.H. Freeman, $16.96) Nelkin demonstrates how the very different cultures of scientists and reporters conflict, unintentionally producing a distortion of scientific progress -- either an overstated "breakthrough" or an overstated calamity -- rather than the slow accumulation of data that is science's business. She faults both scientists (for promoting their own work to get further funding or for avoiding the press altogether) and journalists (for hyping stories and idolizing scientists) but offers little in the way of prescription for better science reportage. As a science writer I find Nelkin's analysis disheartening and not totally on mark (I don't know any self-respecting science writer who uses the word "breakthrough" for example, despite Nelkin's claims). Nevertheless, she raises important concerns for consumers of scientific information, and this book should -- despite its dry style -- be read widely.

Wray Herbert, managing editor of Psychology Today magazine, frequently reviews science and medicine books for Book World.