A desired goal of either ray acoustics or normal‐mode analysis in underwater sound propagation problems is the prediction of the intensity at some field point. The ray‐bundle approach in ray acoustics is a technique which has been employed to predict intensity. By formulating the ray‐path problem in terms of a differential equation, rather than in terms of a range integral, a second differential equation can be obtained which describes the continuous variation of intensity along the ray. An extension of the calculation of intensity to allow for variation of the speed of sound with range as well as depth is easily accomplished. It is shown that, for the intensity function to be continuous, a sufficient condition on the velocity function is that it be twice continuously differentiable except at caustics where the theory breaks down.