15The instrument appointing a proxy shall be in writing under the hand of the appointer, or where the appointer is a corporation under their common seal, or where the appointer is a body of persons unincorporate under the hands of three directors or other persons having the direction or management of the undertaking or business carried on by such body of persons ; and every such instrument shall be attested by a witness, and may be in the form M. in schedule IV. to this Act.