Section 29. Whoever disseminates any matter which is obscene, knowing it to be obscene, or whoever has in his possession any matter which is obscene, knowing it to be obscene, with the intent to disseminate the same, shall be punished by imprisonment in the state prison for not more than five years or in a jail or house of correction for not more than two and one-half years or by a fine of not less than one thousand nor more than ten thousand dollars for the first offense, not less than five thousand nor more than twenty thousand dollars for the second offense, or not less than ten thousand nor more than thirty thousand dollars for the third and subsequent offenses, or by both such fine and imprisonment. A prosecution commenced under this section shall not be continued without a finding nor placed on file. It shall be a defense under this section if the evidence proves that the defendant was a bona fide school, museum or library, or was acting in the course of his employment as an employee of such organization or of a retail outlet affiliated with and serving the educational purpose of such organization.