Section 28. Whoever disseminates to a minor any matter harmful to minors, as defined in section thirty-one, knowing it to be harmful to minors, or has in his possession any such matter with the intent to disseminate the same to minors, 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 in any prosecution under this section that the defendant was in a parental or guardianship relationship with the minor. It shall also be a defense in any prosecution 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.