By JAMES L. GEEENLEAF,
ADJUNCT PROFESSOR OF CIVIL ENGINEERING IN COLUMBIA COLLEGE.
MOST subjects of analysis can be studied both in quantity and quality, and light is no exception to the rule. Color as a quality of light has always been a popular study. A vast deal of experiment and attention has been given to the harmonics of color, and many who make no claim to scientific attainment are well versed in it. Every child knows the colors, but the expression "quantity of daylight" has a strangeness and a vagueness about it that are not felt by children only. This is largely because there is such a wealth of daylight about us. "Silver was nothing accounted of in the days of Solomon." Light is not measured, because it is lavished upon us by an unstinting hand. But light can be measured. Although intangible, it has quantity — quantity of effect, if it may be so expressed. If this seems fanciful, it may be remarked that there is nothing fanciful about the measurement of heat, and yet the case is quite analogous. The thermometer is simply a contrivance for measuring variation of intensity in heat. Quantities of heat effect are continually being estimated, for economy in its use is of prime importance. Light, on the contrary, is ready at hand. Ordinarily objects are flooded with a brilliancy of daylight which is as free as air. There can scarcely be need of economy with light when the world is floating in it. But there is nothing impossible in its measurement. Possibly, if the sun were less prodigal in pouring out his rays upon the earth the measurement of daylight would be a more common operation than at present. Every means would be taken to utilize it without waste. We would see the owners of buildings making careful estimates of the light belonging to their properties — even dividing it into lots and renting them separately. At the least, we would see them more jealously than now defending their light from obstructions built around them.
Windows are the natural and all-important resource of the architect. All the light which enters the building must pass through them. It is a very patent fact that the larger they are, and the more numerous, the greater the total amount of light which enters. It is not as widely appreciated, however, that there are other conditions affecting the amount and quality of the entering light that are sometimes ruling in their effect.
Looking from a window, one sees a variety of surfaces — sees them because of their reflecting different intensities and colors of light. If asked to classify them according to relative reflecting