J. W. Draper—Phosphorograph of a Solar Spectrum. 175 
plates receives a full effect by an exposure of less than one 
minute. 
But all kinds of phosphori will not thus affect a photographic 
tablet: there must be a sympathy between the phosphorescent 
and the photographic surfaces. Thus a phosphorus emitting a 
yellow light will not affect a photographic preparation which 
requires blue or indigo rays. This principle I detected many 
years ago. In my memoir on phosphorescence (Phil. Mag 
to make that interval between the two moments as short as 
possible. 
