THE increase in land values which accompanies the growth of any city results in less space between buildings, which are at the same time, built higher than before. By consequence, it has become a serious problem to secure sufficient natural light, especially on the lower floors in crowded districts. A similar difficulty arises in large factory buildings in more or less isolated locations. With a view to securing all the light possible, the size of the lower floor windows in cities has been increased, and the walls of light shafts painted white. Now, however, it has been scientifically demonstrated that the effectiveness of the available light can frequently he more than doubled by the use of some sort of window glass which will deflect the light in directions where it will do the most good. Experiments in this rearrangement of light have been made by Professor C. L. Norton at the Massachusetts Institute of Technology, and have shown results of such distinct economic value that it is predicted that the use of some sort of ribbed or prismatic glass, of which there are many makes, will soon become practically universal in factories, stores and all kinds of down town buildings in the larger cities.

Curiously enough, prismatic glass, of one kind or another, has been in use for the diffusion or deflection of light in ships for over fifty years. It was found long ago that a cabin was better lighted by a cone of glass set into the deck than by a plane sheet of the same size, but the application of this same principle to the windows of a building is a matter of very recent date. For a number of years, also, it has been customary to use glass roughened to obstruct vision but still admitting light, but while such glass has really a better distribution of light, the fact was not generally appreciated until Mr. Edward Atkinson of Boston brought the matter to the attention of the physical department of the Institute of Technology.

Mr. Atkinson had found that in some English factories a rough, cheap glass was used which seemed to give better results than a perfectly clear plate glass; and taking advantage of the suggestion, he secured the use of finely ribbed or corrugated glass in some at the factories in this country. Although personally convinced that these practical tests were a success, Mr. Atkinson found that his proposition was greatly discredited, especially by men of scientific training. The hopelessness of trying to get something for nothing, that is, to get a sheet of glass to throw into a room more light than fell upon it, appeared so plain even to Professor Norton, that he made all his preparations to measure not a gain but a loss of light in using a roughened or corrugated glass. He was, therefore, surprised when the photometric measurements showed that in some cases the effective light was increased more than 50 per cent., necessitating the rebuilding of his apparatus to gauge so unexpected a result.

The angle at which the light from the sky strikes the window is a most important factor in all the experiments made. In the case of an isolated building the light from the sky strikes the windows almost horizontally, but if another building is erected opposite a window. the sky angle becomes more acute with the nearness and height of the obstruction. When the rays of light are nearly parallel to the window pane, as in the case of or window opening upon the light well of a six story building, the chief strength of the light is upon that part of the floor nearest the window, and below it, while the upper and farther portions of the room are left in gloom.

The effect of the prismatic or ribbed glass is to divert these rays of light from their original direction, so that the light shall be thrown in some other particular direction or diffused through the whole apartment.

The first tests were made by Professor Norton in the physics lecture room in the Walker Building of the Massachusetts Institute of Technology. The room is 53 feet deep and 40 feet wide, with a sloping floor. In the middle of the west side, at a height of 8 feet above the floor, two openings 12 inches square were left in a large window.

All other openings by which light might enter were stopped. A shutter was set over each opening, allowing the light to come through one or the other as desired, so that the effect of the difference in the glass of the two windows could be readily noted. A small screen of white paper was arranged upon a system of trolley wires, so that it might be examined at various points, and a similar screen was attached to a radial arm over 15 feet in length for use in the examination of the intensity of the light at different angles. The comparative measurements of light were made by means of a photometer—a combination of lenses and mirrors, by means of which the standard source of light, a porcelain screen in a window, could be seen as if side by side with the screen whose brilliancy it was desired to determine. The data on the varying sky angle were obtained by placing outside the window, at a distance of about five feet, a vertical curtain, which might be raised to cut off the sky light at any desired angle, with side curtains to cut off the light at the ends.

While these technical experiments formed a basis for calculating the respective value of different sorts of glass under various conditions, the whole story is told more graphically by a series of photographs which show the rearrangement of the light resulting from the use of the diffusing or deflecting glasses.

When a room is from 20 to 60 feet deep and has a sky angle of 60 degrees or less, the use of ribbed or prismatic glass results in such a gain in brilliancy as to make a first story room brighter than a second story room glazed in the ordinary way. If the building which obscures the light from the sky can be increased two stories in height, it will be found possible to restore all the original light of a window, thus obscured by the use of ribbed glass; and even to improve its earlier effectiveness by a prism canopy. Rooms with windows opening upon light shafts and narrow alleys with very limited sky, where the available light is now small, may have the light 20 feet back from the window increased 10 or 20 times by using prisms; and by using canopies or prisms it is sometimes possible to strengthen the light from 50 to 100 times.

This means that in offices and stores, where it is now necessary to use gas or electric light all day, the natural light may be increased sufficiently to survey all practical purposes. On the other hand, the use of these ribbed and prismatic glasses will make it possible for buildings to be placed closer together, without loss of their present light, or to lower ceilings, putting six stories in the height of five or very materially to increase the floor areas.

Except where it is necessary to have windows to look out from it is asserted, indeed, that conditions of economy will soon forbid the general use of plane glass. It is interesting to note some of the particular cases in which it is claimed the scientific diffusion of light will prove especially beneficial. Take, for an example, the ordinary type of school room with windows on two adjoining sides. When the sun is shining sufficiently to light the rear of the room the brightness at the front of the room makes shades necessary, which deprives the rear of the room of its proper share of light. When the sun is not shining directly upon the windows only the front of the room is properly lighted. The result is that the back of the room is insufficiently lighted at all times.

While it hardly practical to glaze the windows so that the pupils near or far from them will receive exactly the same strength of light, the ratio of intensities may readily be changed from 20 to 1, to 2 to 1—a most important sanitary improvement concerned in these experiments.

Practically, the diffusion of direct radiant heat from the sun or from the sky follows much the same lines as the light, but as comparatively little heat enters a room this way, the main effect of the diffusing glasses is to prevent local overheating of room which are exposed to direct sunlight. It is generally deemed to be inexpedient to glaze windows wholly with diffusing glass, as such a room gives to many people a kind of "shut in" feeling, which is distinctly oppressive, and as a result it has been customary to glaze the lower panes of a window with plane glass, as with the diffusion of the upper sash only a sufficient increase of effective light may usually be obtained. But, of course this is a matter depending on expediency and individual tastes, except that it would hardly seem possible for any school board to wish to carry science so far as to shut out the glimpse of the sky even from the idlest and most inattentive schoolroom.