6. (1) If any Inspector or medical practitioner finds-(a) that a child brought for vaccination has already had small-pox, or (b) that a child who has been three times unsuccessfully vaccinated is insusceptible of successful vacci-nation, he shall deliver to the parent or guardian of such child a certificate under his hand, according to the form in Schedule B hereto annexed, or to the like effect. (2) If the Superintendent is satisfied that such child has already had small-pox or is insusceptible of successful vaccination, he shall endorse such certificate. (3) Such endorsement shall operate as an exemption from liability to vaccination,- (i) in case (a) in sub-section (1)- absolutely, and(ii) in case (b) in that sub-section-for a period of twelve months. (4) Upon the expiration of the said period, the parent or guardian of such child shall forthwith cause the child to be vaccinated again; and if an Inspector or a medical practitioner finds after two further unsuccessful vaccinations that the child is insusceptible of successful vaccination, he shall deliver to the parent or guardian a further certificate under his hand, according to the form of Schedule B hereto annexed, or to the like effect; and if the Superintendent of vaccination be again satisfied that the child is insusceptible of successful vaccination, he shall endorse such certificate and such endorsement shall operate as an absolute exemption from liability to further vaccination.