In the second place, the same property, as already stated, furnishes us with the means of tracing the curve by finding successive points therein. Draw a line _a b_ perpendicular to the axis OX, and the position in this line, of a point _p_ through which the curve passes, is easily found thus: Describe from F the focus as a centre with a radius equal to the perpendicular distance O _d_ of the line _a b_ from the directrix AB, a circle cutting the line _a b_ in two points _p_ and _p′_; then both these points are in the curve. By repeating the same process, any number of points in the curve may be obtained. [Illustration: Fig. 24.] Lastly, from the equation to the curve, the length _y_ of any ordinate may be computed, in terms of _m_ its principal focal distance, and x its abscissa, by the simple expression,-- _y_ = √(4 _m_ _x_). It is easy to see, that if this curve revolve about its axis, it will generate a parabolic conoid, which we may conceive to be concave or convex, as we please. If the surface be concave, we obtain the mirror of which we are in search; for every principal section, or that passing through the axis of such a mirror, will necessarily possess the same properties as that of the plane curve, and will each have a focus meeting in one and the same point; the union of all these sections will therefore form a mirror capable of reflecting, in a direction parallel to the axis and to each other, all the rays of light which fall on its surface. ~Divergence of Paraboloïdal Mirrors.~