PLANE AND SOLID ANGLES If two mutually perpendicular lines be drawn through the center of a circle, four right angles are formed, each subtended by one quarter of the circumference. Again, each of these quad rants may be halved or further subdivided at will. If now three planes mutually perpendicular be passed through the center of a sphere, eight equal solid angles are formed called octants, each subtended by one eighth of the spherical surface. If further a circle be drawn on the surface of the sphere equal in area to one of the octants, this circle may be said to subtend a solid angle at the center equal to one eighth of the sphere, and the solid geometrical figure thus defined will evidently be a circular cone with its apex at the center and having a spherical curved base. From this it is seen at once that, if straight lines be drawn from any point in space to every point of the outline of some subject as a door or window, the latter will subtend at the point a definite measurable part of the total solid angle subtended by a spherical surface drawn about that point. Evidently as the point ap proaches the object the solid angle subtended will increase.
If the apex of the cone of light subtended by the door or window in the foregoing illustra tion be located upon some light sensitive sur face, as that of a silver emulsion, then the greater the convergence of light from that source, the nearer the emulsion to the window or door, the less time will be required to produce a stated chemical change. Now as every object about us, even one so indefinite as the sky, emits light capable of effecting this chemical change to some degree, this property is evidently a quantitative one and as will be shown can be measured in simple units. This property has already been termed actinicity.
In taking up the discussion of the unit of actin icity it must be understood that the purpose of this system is not to suggest a primary light source of unit power as a standard but to fur nish a practical means of measuring and express ing in simple numbers the intrinsic actinicity of all light sources, as the sun, the sky, flames and all visible surfaces about us in nature.