[Illustration: _figure 5._] [Illustration: _figure 6._] [Illustration: _figure 7._] [Illustration: _figure 8._] The _axis_ of a lens is a straight line drawn through the center of its spherical surface; and as the spherical sides of every lens are arches of circles the axis of the lens would pass through the centre of that circle of which its sides are segments. _Rays_ are those emanations of light which proceed from a luminous body, or from a body that is illuminated. The _Radiant_ is that body or object which emits the rays of light--whether it be a self-luminous body, or one that only reflects the rays of light. Rays may proceed from a Radiant in different directions. They may be either parallel, converging, or diverging. _Parallel_ rays are those which proceed _equally distant_ from each other through their whole course. Rays proceeding from the sun, the planets, the stars, and distant terrestrial objects are considered as parallel, as in fig. 6. _Converging_ rays are such as, proceeding from a body, approach nearer and nearer in their progress, tending to a certain point where they all unite. Thus, the rays proceeding from the object AB, (fig. 7.) to the point F, are said to converge towards that point. All convex glasses cause parallel rays, which fall upon them to converge in a greater or less degree; and they render converging rays still more convergent. If AB, fig. 7. represent a convex lens, and H G I parallel rays falling upon it, they will be refracted and converge towards the point F, which is called the _focus_, or burning point; because, when the sun’s rays are thus converged to a point by a large lens, they set on fire combustible substances. In this point the rays meet and intersect each other. _Diverging rays_ are those which, proceeding from any point as A, fig. 8, continually recede from each other as they pass along in their course towards BC. All the rays which proceed from near objects as a window in a room, or an adjacent house or garden are more or less divergent. The following figures show the effects of parallel, converging and diverging rays in passing through a double convex lens. [Illustration: _figure 11._] [Illustration: _figure 9._] [Illustration: _figure 10._]