where 
is some suitable parameter, then the actual path (or paths) will be determined by finding the value (or values) of for which 
[P6.2]
states: that if a light ray passes from one fixed point to another fixed point, then the time taken to traverse the actual path will, to a first approximation, be equal to the time taken for light rays to traverse adjacent paths. That is, the time taken to traverse the path will be stationary with respect to small variations in the path. [P6.2]
means, in mathematical terms: that if the journey time along conceivable rays from one fixed point to the other is where is some suitable parameter, then the actual path (or paths) will be determined by finding the value (or values) of for which [P6.2]
means, in physical terms: that light travels along the path which is locally of least time. [P6.2]
permits deduction: of all the basic rules and principles of geometrical optics. [P6.2]
Copyright 1997, The Open University