The office of a physician or surgeon, as defined within N.J.S.A. 45:9-5.1, as amended, or dentist residing on the premises, provided that not more than two persons not residing in such dwelling are employed in each office. Such persons may be nurses, receptionists, secretaries or like assistants. Not more than 1/2 of the floor area of one story of the dwelling may be devoted to the uses permitted in this section.