The board investigator shall immediately investigate the complaint; and upon completion of his investigation he shall inform the accused optometrist that a complaint has been filed against him and that he is under investigation, advise the accused optometrist of the general nature of the charges, furnish him a copy of the complaint and any evidence supporting it, and afford the accused optometrist an opportunity to respond. Communications and notices to the accused optometrist shall be transmitted by registered or certified mail, postage prepaid, to the last known residence or business address of the licensee.