The amount of glazed window surface admitting light to a classroom or study-hall should be from one-sixth to one-fourth the floor space of the room, and this must be increased if the light is obstructed by neighbouring houses or trees. The light is to be admitted on the left side of the pupils,--all other windows should be counted as ventilators only. Windows facing the children or the teacher are to be avoided. In rooms fourteen feet high a desk twenty-four feet from a window is insufficiently lighted. The larger the panes of glass the better, and the external appearance of windows is to be sacrificed to good lighting. If screens are used to protect the glass from stone-throwing, allowance is to be made for the light the screens cut off. If a room can not have enough light from the left side alone, put the additional windows on the right so that their lower sills will be eight feet from the floor; and be careful in this case that the light from the right is not brighter than that from the left. Windows should have as little space as possible between them to avoid alternate bands of shadow and light. Set them up as near the ceiling as possible, since the higher they are the better the illumination; and they should not be arched at the top. The lower window sills may be about four feet from the floor. When window shades are used to cut off direct sunlight, they should be somewhat darker in colour than the walls. If artificial light is used in boarding-schools in the study-halls, the best light is one that is as near in colour as possible to the white light of the sun, and ample, but not glaring. It should be steady, and it should not give out great heat nor injurious products of combustion. Hence the electric light is the best; after that, gas through Welsbach {206} or Siemens burners. Well refined kerosene oil gives a good light, but it is always dangerous. Acetylene gas is now used in a safe apparatus, and it also is an excellent light. No colour that absorbs light should be used on the walls. Pale greenish gray, nearly white, is the most satisfactory colour. There should be no wall paper, curtains, or hangings of any kind in a school or college building. The wall decorations should be as plain as possible, with no roughened places to catch dust. Stairways are to be well lighted; they should be at the least five feet wide, and have landings half-way between each story. Diagonal or spiral stairways are dangerous. Steps with six-inch risers and eleven-inch treads are the easiest for children, but six-and-a-half-inch risers may be used in high schools and colleges.