Lecture, 'Principal planes and the Gauss theory for paraxial refraction at co-axal spherical surfaces,' by Alexander Eugen Conrady Discusses refraction of light at a single surface. Examines systems of more than two media. He outlines the rules for combining systems and its demonstrates some of its applications, as well as goes into detail about eyepieces. Includes some diagrams which have been coloured in. [1921]