Any county probation officer who shall at any time receive a permanent disability by reason of injury, accident or sickness, physical or mental, which shall permanently incapacitate him from further duty, whether he shall have served as a county probation officer for a continuous period of twenty years or not, or whether he shall have reached the age of sixty years or not, shall, upon the certification of the fact of such disability by three physicians designated as hereinafter provided, be retired upon one-half pay.