Since she was in seventh grade, Linda Jacobs has wanted to become a chemist. Last week the tall l3-year-old moved a step closer to fulfilling her ambition as she stood in a laboratory at Goddard Space Flight Center measuring the silver content of optical fiber.

Jacobs was one of three District girls, among 27 from local schools, who were selected for the l983 Institute in Science and Technology for Junior High School Girls, a two-week program, sponsored by the flight center.

The institute was organized in l978 to steer more young women toward careers in science, mathematics and engineering.

"Girls are not pushed into science. We are not encouraged like boys," said Mary Jo Sharp, manager of Goddard's Federal Women's Program, which set up the event.

This year's participants were chosen from a field of 94 eighth grade applicants from the District and Prince George's and Montgomery counties. They were required to demonstrate an interest in math or science, earn a B or better in a science or math course and compose an essay on why they wanted to participate in the program.

Each was then paired with a female scientist, mathematician or engineer for three-hour "mentoring" sessions on two successive days.

Jacobs, who will attend Georgetown Visitation in September, and Valda Norris, l3, another District participant, were placed with mentor Kate Smyth, a ceramics engineer.

They observed Smyth, whose specialty is the scientific study of "high-tech, high-temperature materials," experiment with how light is guided through optical fibers.

Smyth explained how light waves are first created in a bell jar encaged in a huge metal cylinder. Temperature, time and the electronic field are then varied, with the results recorded at timed intervals. Both girls assisted with the recording.

Smyth praised her two assistants and the efforts of the institute. "When I went through junior high school, it was rare to meet a woman scientist. Your only role model would be a man," she recalled.

Norris, who will enter Banneker High School in the fall, said she wants to be a chemical engineer. Asked why, she replied matter-of-factly, "I didn't think being a teacher or a nurse would be as challenging."

The mentors' subject matter varied from the study of the distribution of moon craters to locating sun spots with a telescope, working with a laser and reading weather maps.

Group activities included hands-on computer operation, construction and flight of model airplanes and a tour of the space center.

After working with the Institute for three years, Sharp said, she has noticed that girls' aspirations have shifted. "Two years ago, all I heard about were doctors, lawyers and veterinarians. Now, they're thinking mostly about computers," she said.

Institute coordinators at Goddard said they hope other institutions will follow their lead and establish programs to encourage girls to seek careers in science because the Goddard program has limited capacity and can take no more than 30 participants each summer.

At the closing ceremonies this week, 17-year-old Claire Heerwagen, a l980 graduate of the program, will address the group.

Heerwagen, a college-bound senior from Ft. Washington, Md., said she credits the Institute with influencing her decision to pursue a career in medical technology.

"More and more women are realizing that they have brains that can work logically," she said.