Dr. Gilbert Forbes, research fellow in Legal Medicine, is working as an observer with police investigating the fatal stabbing Friday night of 21-year-old Ethel Ellard, pretty Arlington brunette.

The department of Legal Medicine is taking no special part in the investigation outside of Forbes' work, according to Dr. Richard Ford, assistant professor of Legal Medicine. He said last night his department has only been giving advice, as it usually does when tough murder cases come up.

Forbes, who is a teacher of legal medicine in Sheffield University in England and deputy coroner for the city of Sheffield, came to this country to see how American police track down killers.

Ford said neither he nor Forbes could comment on the case because of professional courtesy. But he declared Boston is a good place for Forbes to study murder because there are "more murders even in staid Massachusetts than in all England."