Benedict J Fernandez's "In Opposition: Images of American Dissent in the Sixties" is a kind of pictorial primer of protest, both for and against whatever the cause-Vietnam, fair housing, white or black power , student sit ins. Convinced that people who feel strongly about anything should make their voices heard in "the marketplace of ideas"- the variety of demonstration techniques, indoor and out, this restless decade has produced- Mr. Fernandez has made a specialty of being around when they do and taking pictures to spread the word and the sense of indignation further through exhibitions and publications.