[Illustration: FIG. 382.--_C_ and _S_ are at conjugate foci.] It will be helpful to compare the images formed of a candle by an _aperture_ and by a _convex_ lens. Rays of light from each point of the luminous body pass through the aperture in straight lines and produce upon the screen a lighted space of the same shape as the candle. This image is rather _hazy_ in outline. Each cone of rays from luminous points of the flame is brought by the lens to a focus on the screen, producing a _sharp image_. It is the converging power of convex lenses that enables them to produce clear images. [Illustration: FIG. 383.--Construction of a real image by a convex lens.] =387. The Construction of Diagrams to Represent the Formation of Images by Lenses.=--Just as the earth has an axis at right angles to its equator to which are referred positions and distances, so a lens has a _principal axis_ at right angles to its greatest diameter and along this axis are certain definite positions as shown in Fig. 383. Let _MN_ be the _principal axis_ of a convex lens, _P_ and _P´_ are _principal foci_ on either side of the lens, _S_ and _S´_ are _secondary foci_. These are at points on the principal axis that are twice as far from _O_, the center of the lens, as are the principal foci. In the formation of images by a convex lens, several distinct cases may be noticed: (A) If a luminous body is at a _great distance_ at the left, its light is brought to a _focus_ at _P_, or its _image is formed at P_. (B) As the _object approaches_ the lens the _image gradually recedes_ until the object and image are at _S_ and _S´_, _equally distant from O and of equal size_ (as in Fig. 383). The object and image are now said to be at the _secondary foci_ of the lens. (C) As the _object moves from S to P_ the image recedes, rapidly increasing in size until (D) when the object is at _P_ the rays become parallel and no image is formed. (E) When the object is between _P_ and the lens, the rays _appear to proceed from points back of the object_, thus forming an _erect, larger, virtual image_ of the object. (See Fig. 384.) This last arrangement illustrates the _simple microscope_. With a concave lens but one case is possible, that corresponding to the one last mentioned with convex lenses; since the rays from a body are divergent after passing through a concave lens they appear to proceed from points _nearer_ the lens than the object and hence a _virtual, erect, smaller image_ of the object is formed. This virtual image may be seen by looking _through_ the lens toward the object. (See Fig. 385.) [Illustration: FIG. 384.--Construction of a virtual image by a convex lens.] [Illustration: FIG. 385.--Construction of a virtual image by a concave lens.]