An Optical Representation of Non-Euclidean Geometry

LET us suppose Euclidean space to be filled with a medium of variable refractive index. Then to an observer in that medium the curved path of a ray of light will present all the appearances of a straight line, and, further, if the observer estimates the distance between two points by the time light takes to pass between them, this path will appear to be the shortest distance between the two points.