Visibility

THE factors which determine the ability of the eye to distinguish an object from its background are not very well known for many of the conditions met with in practice. Our practical experience has taught us that if we wish to see properly under any given condition, the illumination must not be less than a certain amount, and we have been content to provide the necessary illumination without having any very clear idea what are the aids and hindrances to good seeing. A discussion on some aspects of the subject of visibility was initiated at the Illuminating Engineering Society, on April 27, by Messrs. Paterson and Dudding, of the National Physical Laboratory; and many speakers made interesting-contributions to it.