bright light the iris diaphragm contracts, reducing the size of the pupil and cutting out much of the light which would otherwise enter the eye. In the presence of a dull light the pupil enlarges, allowing a great amount of the light, such as that falling upon a book, to enter the eye and to stimulate the retina. The iris is a wonderful device, but can not in diverse illuminations perfectly equalize the amount of light entering the eye. The pupil expands inversely as the square root of the illumination. Thus if the actual illumination of the book increases ten-fold, the amount of light falling upon the retina is increased but little over three-fold. Even a twenty-five candle light sends but five-fold as much light into the eye as a single candle-power. A single candle-power seems sufficient and ten-candle power is not too much. This ability of the retina and of the iris to deal successfully with lights of such different intensities is a most useful and necessary characteristic. Unfortunately, however, the actual diversities of intensities of lights used for reading are far beyond any for which the eye can adapt itself.
Variations in the Amount of Daylight
We are in the habit of thinking of the light received from the sky—the daylight—as almost a fixed quantity during the hours from 9 in the morning till about 4 in the afternoon. The darkness preceding a storm and the occasional dark days are of course not forgotten, but, in general, daylight for the hours mentioned is thought of as at least fairly constant. To test this point observations were made at 9 a.m., 12:30 p.m., and 4:30 p.m., daily for five and one half days a week for 22 months. These tests were made in the Chicago laboratory of the American Luxfer Prism Co., and under direction of Professor Olin H. Basquin, of the Department of Physics of Northwestern University. Inasmuch as the amount of sunshine and general illumination in Chicago is almost exactly the average for the United States, these results may be regarded as typical for the whole country with the exception of such dark cities as Seattle or such light ones as Phcenix, Measurements were made of the amount of light coming through a square foot of clear glass placed horizontally in the roof of the observation building. The illuminometer was placed so far below the opening in the ceiling that the direct rays of the sun could never reach any part of the recording apparatus. The light thus measured was diffuse daylight received from the zenith of the sky. Taking the average illumination for the 22 months at 12:30 as the standard, it was found that the illumination at 9 a.m. was but 67 per cent, as great as that of mid-day. Again the illumination at 4:30 p.m. was but 27 per cent, as great as that at 12:30. Expressed in other terms, we see that the available light at 4:30 is approximately but one fourth that of noon and the light at 9 o'clock but two thirds that of noon. These
