Physical unfitness or incapacity for further duty of any sheriff’s employee shall, for all purposes of this article, be established and determined by a board of three physicians who shall be designated for that purpose, as follows: One by the sheriff of the county; one by the sheriff’s employees of such county or a majority of them, and one by the county treasurer of such county; the three physicians so designated shall examine sheriff’s employees applying for retirement upon half pay because of physical unfitness or incapacity for further duty, and if they, or a majority of them, find him physically unfit or incapacitated for further duty, they, or a majority of them, shall make and sign a certificate to that effect and file the same with the county treasurer, and thereupon the applicant shall be retired upon half pay.