Pure and Applied Optics. — Optics falls naturally in three divisions; pure optics, applied optics and cosmical optics. Pure optics treats of the laws and phenomena connected with the emission, propagation and absorption of light, from either the physical or mathematical standpoint. Applied optics deals with the applications of optical principles to optical observations and measurements, hence with the design, use and properties of all kinds of Optical instruments and their accessories. Subdivisions of applied optics deal chie?y with the direction of the light, its intensity, quality and state of polarization. Photometry, spectrophotometry, colorimetry, Optometry, photography, Optical pyrometry, polarimetry, interferometry and many other subjects are included in applied optics. Cosmical optics deals with terrestrial, planetary, solar and stellar optical phenomena. This introduction to applied optics is a summary of those parts of pure optics of most interest to the technologist.