Where a method of analysis or examination has been prescribed for the analysis or examination of a particular kind of substance or plant, an analyst, either for the complainant or for the defendant in proceedings under this Act, shall, in his certificate of analysis or examination, declare that he has followed the prescribed method in his analysis or examination; but evidence is admissible on the part of the defence of an analysis or examination made by a method other than the prescribed method and to show that the prescribed method is not correct.