The main point is to secure that the house shall receive ample =light= and ventilation. In calculating the =amount and intensity of sunshine= which a house built on a given site will secure the variations according to season must be remembered. The direction (orientation) of the sun is the same all the year through; but the altitude of the sun varies with the latitude. Thus in a house facing directly south in the latitude of the south of England the sun’s altitude at noon on the 21st of December is 15° 4´, on the 21st of June 62° 4´. A ray of light entering the highest point of a window facing south at each of these seasons will illuminate a much larger part of the room in summer than in winter. Not only so, but inasmuch as it enters the room more nearly vertically it is more powerful than when entering at an angle more nearly approximating a horizontal direction, in accordance with the general law that the intensity of illumination falling on a horizontal surface (as the floor of a room) is inversely as the square of the width of the area embraced within the same angle of incidence of light. In houses in a street the angular aperture through which light enters is greatest in the upper stories. It may be increased (_a_) by increasing the height of rooms; (_b_) by carrying the window heads nearly to the level of the ceiling; and (_c_) by avoiding the proximity of other buildings which would impede the access of light. Fig. 36 shows the importance of the last consideration. This represents a three-storied house in a street, of which the opposite house L is of the same height. It will be observed that each room is divided into two regions of different degrees of illumination by a plane Lm, formed by a line connecting the ridge of the roofs of the houses on the opposite side of the street with the interior surface of the rooms and touching the uppermost point of the window in transit. Below this line there is “sky-light” sufficient in quantity; above this line light is insufficient in amount and is diffused and reflected. The area receiving sufficient light increases from the ground floor upwards. We have already seen that its intensity similarly increases in the higher stories, the rays of light being more nearly vertical in these. [Illustration: FIG. 36. SHOWING VARIATIONS OF ILLUMINATION IN DIFFERENT STORIES.] The amount of sky-light visible can be expressed in terms of the _angle of aperture_, i.e. the arc of sky visible at any given point _a_ in the room. Thus in Fig. 36 the triangle of aperture _bac_ is greater than _b´a´c´_, and this greater than _b´´a´´c´´_. The sides of the angle of aperture, it will be seen, are formed by drawing one line from the point a to c, which, if prolonged, would touch L, and another line to _b_, which passes through the highest point of the window.