Natural light should be secured if possible for every room. Windows should be frequent and extend well up toward the ceiling terminating in a straight line so as to afford large supply of light from the top. Windows like those in an ordinary house or office building, coming within two or three feet of the floor are more satisfactory both for inside and outside appearance than those which leave a high blank wall beneath them. From the street a blank wall has a prison-like effect; on the inside it cuts off communication with the rest of the world and the impression is unpleasant. The proper object of library windows six or eight feet above the floor is to allow unbroken wall space for book shelves beneath them. There is no serious objection to this at the back of the room or sometimes at the sides of the house where the windows are not conspicuous from the street, but every room of any size, if it is next to the outer wall, should have windows to look out of on at least one side. A book room at the back of a building may secure excellent light from side windows eight feet above the floor with lower windows at the back. The lighting of large interior rooms is often a difficult problem. Light will not penetrate to advantage more than 30 feet. Skylights, domes and clerestory windows are used. In the case of the dome or clerestory the room to be lighted must be higher than those immediately surrounding it. The clerestory plan with upright windows is most satisfactory when available, being cheaper and giving better security against the weather than the skylight. In a large building with interior courts, the lower story of the court is sometimes covered with a skylight and used as a room. This appears in the plans for the New York Public and the Utica Public libraries. Skylights must be constructed with special care to protect rooms against the weather. The problem of light is peculiarly difficult in the crowded blocks of cities. A library front may sometimes touch the walls of adjoining buildings so that light can enter only from the front and rear. If extending more than 40 feet back from the street, it will be necessary to narrow the rest of the building so as to leave open spaces on each side, or to introduce a little light by the device of light wells. Occasionally a large city library is found on the upper floors of an office building, where light and air are better than below, and the cost of accommodation is less. The use of elevators makes this feasible. _Shelving._