An Appeal to Rationality...

Eckhard Hess, 69, professor of biopsychology at the University of Chicago, discovered an eye reflex that can be used to determine whether people like or dislike what they are looking at.

His research led to a field known as pupillometrics, based on the theory that when people see something they like or find interesting their pupils dilate, but when they view something disagreeable, their pupils get smaller. It has been used by market researchers as a better index of likes and dislikes than the testimony of individuals.

Graveside services will be held at 2:30 p.m. Sunday in Old Trinity Church Graveyard, Church Creek, Dorchester County, Md. He died Friday in Dorchester General Hospital in Cambridge, Md.

Mr. Hess accidentally discovered the pupil response in 1959. He was in bed looking at some beautiful pictures of animals. His wife commented that something must be wrong with him because his pupils were too large. He couldn`t sleep that night trying to figure out what had happened. The next day, he submitted a series of photos to a student. They were dull landscape scenes with the exception of pin-up picture of an attractive woman.

''I noticed his pupils suddenly pop open,'' he later said. ''When I looked at the picture he was looking at, it was the pin-up.''

The right man had discovered the reflex. At the U. of C., he had the laboratory, the research tools and the available subjects to test his theory. Experiments showed that women`s pupils dilate at the sight of male nudes, but not as much as men`s pupils when viewing female nudes.

He also found that among 34 students and faculty who said they supported Lyndon Johnson over Barry Goldwater for president in 1964, the pupils of a third of them got larger when they saw a picture of Goldwater than of Johnson. Mr. Hess inferred that they were stifling their support of Goldwater in a more liberal environment.

Market researchers, specifically a New York firm called E.Y.E., developed the theory into a system of eye tracking that measures pupil dilation and contraction and the position of the eye to test the subject`s response to various parts of projected ads.

He was asked many times to use his techniques in professional lie detection, but refused out of fear that they might be abused.

A native of Bochum, Germany, he came to the United States in 1927. He received a Ph.D. from Johns Hopkins University in psychology in 1948.

Other experiments for which Mr. Hess was noted involved animal psychology. Animals he studied included wolves, guinea pigs, mourning doves and a variety of other creatures that ranged from termites to fish. His specialty was a behavior known as ''imprinting,'' an early attraction to one`s own kind.

It can also happen when an animal such as a newly hatched duck follows the first object it sees moving, such as a rubber ball, believing it to be its protector and mother.

Among the books he wrote were ''Imprinting and the Tell-Tale Eye.''

Survivors include his wife, Dorothea.