The first term is the straight path travel time, the second term is the extra geometric path, and the third is the gravitational delay. Make the triangle approximation that α α ( z ) = θ θ − − β β for the path between the observer and the lens, and α α ( z ) ≈ ≈ ( θ θ − − β β ) D d D d s for the path between the lens and the source. The geometric delay term becomes