Who? Discovered Coddington's Equations?
In these formulae, s and t represent the distances from the point of incidence of the traced ray to the sagittal and tangential object or image, measured along the traced principal ray, and r is the radius of curvature of the refracting surface. Angles I and I' are the angles of incidence and refraction of the principal ray while n and n' are the refractive indices of the object and image spaces.
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