Washington.

Nearly two centuries ago, the British astronomer Sir William Hershel was using a prism to separate the colors of the visible part of the spectrum -- the colors of the rainbow.

Sir William, best known for his discovery of the planet Uranus in 1781, was also using a thermometer to measure the heat energy emitted by each of these colors; violet, indigo, blue, green, yellow, orange and red.

Curiously, the temperature rose even when Herschel moved the thermometer "beyond the red" -- and beyond the capability of his eyes to see it. This invisible color became known as "infrared."

Along with ultraviolet, X-ray and radio astronomy, observations of the sky in the infrared have become a major exploratory tool in recent years.

So much so, in fact, that a group of the nation's most distinguished astronomers last week assigned top priority in the 1990s to NASA's planned Space Infrared Telescope Facility (SIRTF) which, they said, would be "almost a thousand times more sensitive than Earth-based telescopes operating in the infrared."

Why this interest in the infrared?

To begin with, everything emits infrared radiation, including stars, galaxies, dogs, cats and human beings. In addition, infrared radiation is a major part of the "energy budget" of the universe. And all of this energy budget, vibrating across the breadth of the electromagnetic spectrum from gamma rays to radio waves, provides scientists with the data that help them determine the origin, evolution, size, chemical structure, temperature and movement of celestial objects.

More specifically, most of the known mass in galaxies is in the form of relatively "cool" stars that are brightest in the infrared, the astronomers pointed out. In addition, the expansion of the universe shifts radiation from primeval (most distant) objects in the cosmos from the ultraviolet and visible bands into the infrared. And atoms and molecules that make up cosmic objects have rich infrared spectra, or signatures, that can be used to probe the density, temperature and other properties of these objects.

Furthermore, the SIRTF is the only one of NASA's "Four Great Observatories" that has not yet been funded by Congress. The first of the four, the Hubble Space Telescope is already aloft; the Gamma Ray Observatory is scheduled for launch this year, and work on the Advanced X-Ray Astrophysics Facility is under way.

All this adds up to a fascinating future of discovery, not only for professional astronomers but for a host of amateur astronomers and just plain inquisitive people who wonder how it all began and where it's going to end -- if ever.

With the remarkable advances in rocketry and the computer sciences since World War II, the price tag for many -- but not all -- of these astronomical studies is considerable.

As is the case with the sky's X-ray and ultraviolet radiation, cosmic energy in the wavelengths of the infrared does not readily penetrate the atmosphere. That saves all of us from being fried but calls for the placement of infrared telescopes on high mountains or, better yet for some research, above all of the atmosphere, which is what NASA intends for the $1.3 billion SIRTF.

Beyond these considerations is the remarkable success in 1983 of a less sophisticated infrared space observatory, the Infrared Astronomy Satellite (IRAS), which was a joint project of the United States, the Netherlands and Britain.

IRAS found what appeared to be the beginnings of a planetary system around Vega, the third brightest star in the heavens; detected a region of the sky where stars similar to the Earth's sun are being born; observed 20,000 galaxies; helped spot five new comets, and discovered three narrow rings of dust within the solar system -- possibly the debris of asteroid collisions in a region of the solar system between the orbits of Mars and Jupiter.

The assignment of top priority to the SIRTF, which scientists hope will locate newly forming stars and galaxies, was the decision of the Astronomy and Astrophysics Survey Committee chaired by Dr. John N. Bahcall, director of the School of Natural Sciences at the Institute for Advanced Study in Princeton, N.J.

The 15-member panel, operating under the auspices of the National Research Council, an arm of the National Academy of Sciences, also favored the proposed ground-based 8-meter infrared telescope on Mauna Kea, Hawaii, for studying the origin and evolution of planets, stars and galaxies; a so-called millimeter array instrument, consisting of 40 transportable radio antennas that would study solar flares and probe the cores of infrared-lumnious galaxies; and an 8-meter telescope in the Southern Hemisphere to view the sky from the underside of the planet.

Referring to more prosaic priorities, the panel urged the National Science Foundation, the federal agency that supports a broad field of research, to bolster support for the "basic infrastructure of astronomy" by increasing funds for individual grants to astronomers, establishing computer networks for the exchange of scientific data and paying for the maintenance and refurbishment of major ground-based facilities.

Dr. Bahcall cited one instance in which an array of radio telescopes could not be safely moved along their network of railroad tracks because the ties holding the tracks in line were deteriorating.

"No prudent businessman would run a business with such a small investment in the infrastructure," he said.

The Bahcall committee's recomendations were based in part on discussions with the White House, Congress, the Office of Management of Budget, the National Aeronautics and Space Administration and scores of other astronomers.

"In all, more than 15 percent of American astronomers played an active role in at least one aspect of this study," Dr. Bahcall wrote in a preface to the study, entitled "The Decade of Discovery in Astronomy and Astrophysics."

"This report is one of the most important things we've done in recent years," Dr. Frank Press, chairman of the National Research Council, said last Tuesday during a discussion of the report at the National Academy of Sciences.

Dr. Bahcall said the "hardest part" of preparing the report was "saying 'no' to many brilliant and carefully planned projects."

"We are taking the heat for our imperfect decisions," he said in the course of Tuesday's discussions at the Academy.

On the other hand, he seemed fascinated by the work ahead and what it will reveal about the universe in the present decade.

"There's much more out there than any of us have dreamed of," he said, speaking in the same vein of such prominent astronomers as Allan R. Sandage, who asked in an interview with this newspaper a few years ago: "What is man's place in the universe?"

"Every atom in your body was once inside a star," Dr. Sandage observed, referring to humankind's apparent emergence from promodial stellar materials.

Dr. Carl Sagan, who fascinated millions with his "Cosmos" television series, followed the same avenue of thought at Tuesday's discussion, noting that the calcium in our bones and the iron in our hemoglobin are the stuff of stars.

And the obvious question in all of these cosmic studies, he said, is: "Where did you come from?"

Albert Sehlstedt, a retired science writer for The Sun, has written about the space program since its inception.