15. In any prosecution under this section,- (a) it shall not be a defence to allege that the person who manufactured or sold the article of food had no knowledge of, and could not with reasonable diligence have ascertained, the nature, substance or quality, of such article or of any article used in the preparation thereof; and (b) the Court shall, until the contrary is proved, presume that any article of food notified under section 7 or specified in sections 8, 9, 10, 11, 12 and 13 or referred to in clause (b) of section 14, or any ingredient of, or anything used in the preparation of, such article, which is found in the possession of any person who manufactures or sells similar articles, has been manufactured by that person or has been kept by him for being used in the manufacture of such article or is for sale by him or has been kept by him for the purpose of preparation of any food referred to in clause (b) of section 14, as the case may be.