All members of the police force of the village shall, before entering upon duty as policemen, take and subscribe an oath of office, substantially in the following form:
I do solemnly swear that I will support the Constitution of the United States and of the State of Illinois, and that, I will faithfully discharge the duties of the office of policeman of the village of River Grove to the best of my ability.
Which oath shall be subscribed and sworn to before some officer authorized by the laws of this state to administer oaths, and the certificate thereof filed in the office of the clerk. (Rev. Code 1939)