In this way are built the stack rooms of the larger modern libraries, and the most compact storage of the books, with ease of access to every part, is obtained. In the new building of the public library at Boston there are six of these book storeys; in the stack rooms, at the library of Congress, Washington, there are nine; but the general rule is to have four. At Boston but little reliance has been placed upon natural light, and it has been thought best to rely upon a plentiful number of electric lamps. In book stores lit only from the roof the light will no more than penetrate one floor, and so light but two storeys; if the gangways are of exceptional width, as in the British Museum, three storeys may be lit, but any greater number must rely upon side lights. If the latter be large and well arranged on two opposite sides, the book store may be 40 feet in width, but the length of the cases will depend greatly upon the size of the room and its lighting.
It has been already recommended that the bottom of the side windows in ordinary rooms should be 8 feet from the floor, so as to allow a continuous run of bookcases round the room, but in book stores with stacks more than two storeys in height this is impossible, and side-windows must be provided for each storey. The stack cases will be placed at right angles to the windows, to allow the light to penetrate between them. The distance between each stack will depend upon the distance the windows are apart, the minimum being 2 feet 6 inches; but this width should only be adopted
