These curves are what are known as the "+conic sections+"--that is, they are the curves found when a cone is cut across in different directions. Their relation to each other may be illustrated thus. If we have a very powerful light emerging from a minute hole, then, if we place a screen in the path of the beam of light, and exactly at right angles to its axis, the light falling on the screen will fill an exact circle. If we turn the screen so as to be inclined to the axis of the beam, the circle will lengthen out in one direction, and will become an ellipse. If we turn the screen still further, the ellipse will lengthen and lengthen, until at last, when the screen has become parallel to one of the edges of the beam of light, the ellipse will only have one end; the other will be lost. For it is clear that that edge of the beam of light which is parallel to the screen can never meet it. The curve now shown on the screen is called a +parabola+, and if the screen is turned further yet, the boundaries of the light falling upon it become divergent, and we have a fourth curve, the +hyperbola+. Bodies moving under the influence of {35} gravitation can move in any of these curves, but only the circle and ellipse are closed orbits. A particle moving in a parabola or hyperbola can only make one approach to its attracting body; after such approach it continually recedes from it. As the circle and parabola are only the two extreme forms of an ellipse, the two foci being at the same point for the circle and at an infinite distance apart for the parabola, we may regard all orbits under gravitation as being ellipses of one form or another.