As the apparent contrast of a square test object situated behind a smoke layer is decreased in laboratory experiments a point is reached at which the object appears as a light patch of unrecognizable shape. On decreasing the apparent contrast a little further the object becomes completely invisible. A study of the conditions under which the apparent loss of definition occurs was carried on simultaneously with the study of total obscuration described in the preceding paper. The optical density of smoke required to produce loss of definition was found to be linearly related to the logarithm of the ratio of object to cloud brightness, in close correspondence with the relation found for total obscuration. Over the range of our experiments the minimum optical density of smoke associated with loss of definition was from 12 to 23% less than that associated with total obscuration, the magnitude of the difference depending on brightness conditions. Since the phenomenon of loss of definition is observable in the presence of a veiling glare without smoke, it is probably of physiological origin.