A Treatise on Optics

aberration achromatism angle of incidence bright caustic centre circle of least coincides colours concave conical surfaces converge convex lens cos² curve cuts the axis diameter direct pencil distance emergence equal equation excentrical eye-glass eye-piece fixed lines focal length geometrical focus glass Hence illumination inches incident directly incident pencil index of refraction lenses Let Q luminous point magnifying power medium object object-glass parallel rays passes pencil incident pencil of light pencil of parallel pencil of rays pencil whose axis perpendicular plane mirror plane surface point of incidence position primary plane principal focus principal plane prism radii radius rays incident reflected or refracted reflexion or refraction refracted pencil refracted rays refracting angle refracting surface refractive index retina shew sin² small pencil spectrum sphere spherical reflector spherical surface supposed telescope thin lens V₁ v₂ αμ