This paper presents a series of curves which are the results of some calculations of the reflecting properties of various models of the ionosphere for radio waves of frequency 16 kc/s. The method of calculation was described in a previous paper (Budden 1955). No attempt is made to deduce a model of the ionosphere capable of explaining all the observations, but the aim has been rather to establish some general principles which may indicate how future theoretical and experimental work should be planned. In most of the calculations it was assumed that the earth's magnetic field is vertical and that the electron collision frequency in the ionosphere is constant. The limitations imposed by these restrictions are discussed. The first half of the paper describes some calculations for a model of the ionosphere in which the electron density increases exponentially with height, and the second half deals with a model having both D- and E-layers. The results in both cases are compared with observations.