Optics in the paper is considered as consisting of three layers: geometrical optics, which gives the geometry of the problem (straight lines equal light rays, spheres equal wave surfaces), diffraction optics, which considers a periodic disturbance of given frequency along selected light rays (a field), and the theory of emission and absorption, which considers the selection of discrete frequencies.
Physics can be built up parallel to it as optics in space-time. The geometry of physics based upon a variation principle contains general relativity as a special case; diffraction physics gives the laws of propagation of electromagnetic energy of all wave-lengths in a well-defined field; and the theory of emission and absorption selects suitable frequencies.
The paper describes in detail the laws of the first level and sketches the basic problems of diffraction physics. The problems of emission and absorption are only mentioned (Einstein effect) and will be treated in detail later on. Electricity and gravitation emerge in this theory as the physical equivalence of the fact that the field equations can be split into a symmetric and an antisymmetric part.
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