This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Fig. 718 shows the angles at which the sun's rays enter the windows on each side of a house standing directly north and south, on June 21 and December 21, the former being distinguished by open and the latter by black feathers to the arrows. From this it will be seen that in mid-winter the south window alone admits direct sunshine. Fig. 719 shows the sun's altitude at noon on the 21st day of each month in the year, from 15° 4' in December to 62' 4' in June.
Figs. 720 and 721 show the incidence of the sun's rays in a room facing south, every two hours from 8 a.m. to 4 p.m. on the 21st days of June and December, in the latitude of London.
Fig. 718. - Sunshine and Aspect.
Fig. 719. The Sun's Altitude at Noon throughout the Year.
Fig. 722 explains the ratio which the intensity of illumination bears to the angle of incidence, and applies alike to the intensity of the sun's rays at different seasons of the year, different hours of the day, and different terrestrial latitudes, and also to the distribution of light in a room at different distances from the window or other source of light. It represents a number of pencils of light, a, b, c, d, and e, each including 10° of arc, proceeding from the same luminous point o, and therefore, under like conditions, of equal illuminative power. But falling on the horizontal surface m 1 at different angles of incidence, they are spread over spaces increasing as the tangents of the angles that the rays make with the perpendicular o m, and the intensity of illumination in the several zones will be inversely as the squares of their width. In this instance the sections f g. i k, and k 1, being (approximately) as 1, 2 and 3 3/4, the relative illumination will be as 12, (1/2)2, and (3/7)2, or 1, 1/4, and 1/12. In the last case the angle made by the rays with the horizontal is 25°. Less, however, than 30° should not be permitted, since the light, even though derived direct from the sun or sky, will then be so dispersed that the end of the room most remote from the window will be inconveniently dark; and in this respect lofty rooms, or, rather, rooms high in proportion to their depth, present a great advantage.
Fig. 720 and 721.- Incidence of the .Sun's rays at Noon in Window facing South between the boon of 8 a.m and 4 p.m.
Fig 722. - Ratio between Intensity of Illumination and Angle of Incidence.
Opposite windows equalize the illumination throughout, and even as regards schools the prejudice against "cross lighting" has little if any justification so long as the light from the right hand cannot be the stronger. Reading, and indeed any work requiring a clear vision, is nowhere so easy and pleasant as in the open air, when the sky is lightly "overcast", and the light seems to come from all sides though from nowhere in particular.
There are two other factors determining the degree and amount of illumination in a room, which, depending on the width of streets and the height of houses, assume great importance in town-dwellings, though they must not be ignored in the arrangement of wings and detached buildings in country houses. No room can be considered properly lighted unless the whole or the greater part of the portion in which the occupants move, receive its light direct from the sky. In towns, and wherever the view of the horizon is obstructed by buildings, trees, or rising ground, the ceiling will necessarily receive diffused and reflected light only; and it is with the aim of aiding reflection that (filings are usually whitewashed. The nearer the tops of the windows approach the ceiling the less will be the depth of the zone of comparative darkness. The admission of strong sunshine may indeed be undesirable, and for libraries, studios, laboratories, and especially for microscope rooms, a north aspect is decidedly to be preferred, provided always that the window-space be ample, and a wide expanse of sky visible throughout the chamber. But in all rooms used for living, reading, writing, needlework, etc., the plane dividing the upper zone of diffused and insufficient from the lower of direct and sufficient light should be above the heads of the occupants, or at least above their work. The distribution of light and shade in the several floors of a house situated in a street which is a little wider than the height of the houses on either side, is shown in Fig. 723; the third and fourth stories are seen to be adequately lighted, but the second and lowest inadequately - especially the ground-floor.
The other factor - which Prof. Forster of Breslau, borrowing a term from optics, calls the "angle of aperture" - is determined by, or is a measure of, the vertical height of the portion of the sky visible at the centre of the floor, or, as I would rather put it, by a person seated at a table in the middle of the room.
Fig. 723 -Light and Shade in the several Stories of a House.
In Fig. 723 it is indicated by the lines f d and g d, drawn respectively from the summit of the opposite buildings, and from the head of the window, to the point in question. In this figure it is seen to be ample on the top floor, leas on the next two, hut non-existent on the lowest, in which the head of the worker is in the zone of shade, and sky-light can be enjoyed only by kitting close to the window. In a base ment room, even this could be obtained only by means of a wide area, if at all.