This bulletin is a summary of about 45,000 observations of daylight illumination made by the United States Public Health Service in a school building in Hagerstown, Md., during the school year 1923-24. The influence of clouds upon the inside illumination, the outside illumination, and the sky brightness has been made the subject of special study. Also, an analysis is made of the ratio of the inside illumination on a desk to the total outside illumination, (due to both skylight and direct sunlight), and of the ratio of the inside illumination to the outside sky brightness; the distribution of the light within the rooms, and the changes in the distribution accompanying changes in other factors or attendant circumstances; and, finally, an analysis is given showing the relationship, under varying cloud conditions, between the inside illumination and the area of the sky vault which is visible from each desk, and a generalized formula for forecasting inside illumination under any set of attendant conditions. At the end of the paper a summary of the conclusions is given.