THE apparently simple question “What is Daylight? “raises at once a host of supplementary questions which are not easy to answer, and a little consideration shows that “daylight “is a word of somewhat indefinite meaning. The paper on which the present article is being written is illuminated by light from a north window. The sky is blue, but flecked with white clouds reflecting winter sunlight. A proportion of the light is, however, coming from the walls of an opposite house, and since this house is flanked by green trees and shrubs, they are also con-tributing their share of reflected light. A spectro-photometric examination of the light would doubtless reveal a somewhat irregular spectral distribution of energy, varying from minute to minute, although the eye registers no marked change in the appearance of the paper. Even after the drastic step of drawing the blinds and switching on the electric light, the appearance will scarcely indicate the tremendous alteration in the nature of the light, at any rate when the eye has been accustomed to the changed conditions. It is not until some effect of simultaneous contrast brings the artificial light into comparison with daylight that the difference between the two is revealed.