a solicitor shall not be required to pay the annual contribution on the issue of the first three practising certificates issued to him after his admission; and

on the issue of the fourth, fifth and sixth practising certificates issued to him after his admission a solicitor shall be liable to pay only one half of the amount of the annual contribution; and

an annual contribution shall not be required to be paid by a solicitor who is employed as a full-time public officer or in the full-time service of a local or public authority and who does not engage in any form of private practice as a solicitor.