Section 76. Every person retired under section seventy-five shall receive an annual pension equal to one half of the compensation received by him at the time of his retirement, to be paid by the county in which he has performed his duties, or if he has performed such duties in more than one county, it shall be apportioned by the superior court among such counties; provided, that no person so retired upon attaining the age of seventy shall be entitled to such pension unless, during the fifteen years immediately preceding retirement, he has given his whole time to the duties of the office of probation officer and has faithfully performed such duties.