No pocket billiard or bowling alley shall be operated in the city by any person except upon the ground floor of a building. Plate glass window shall be in the part of the building facing the street, so that the public shall at all times be able to see within the room. Specifically excluded from the requirement of plate glass windows in the part of the building facing the street of this section are establishments whose principal purpose or business is the operation of any other lawful enterprise and the maintenance of a pool table or tables or the maintenance of a bowling alley is incidental only to the principal purpose of the business.