_Chromatic Aberration_ is another difficulty that opticians have to contend with in the manufacturing of lenses. It will be remembered, that in a former page (14) a beam of light is decomposed by passing through a glass prism giving seven distinct colors--_red_, _orange_, _yellow_, _green_, _blue_, _indigo_ and _violet_. Now, as has been said before, the dissimilar rays having an unequal degree of refrangibility, it will be impossible to obtain a focus by the light passing through a double-convex lens without its being fringed with color. Its effect will be readily understood by reference to the accompanying cut. [Illustration: Fig. 12.] If L L be a double convex-lens, and R R R parallel rays of white light, composed of the seven colored rays, each having a different _index_ of refraction, they cannot be refracted to one and the same point; the red rays, being the least refrangible, will be bent to _r_, and the violet rays, being the most refrangible, to _v_: the distance _v r_ constitutes the chromatic aberration, and the circle, of which the diameter is _a l_, the place or point of mean refraction, and is called the circle of least aberration. If the rays of the sun are refracted by means of a lens, and the image received on a screen placed between C and _o_, so as to cut the cone L _a l_ L, a luminous circle will be formed on the paper, only surrounded by a red border, because it is produced by a section of the cone L _a l_ L, of which the external rays L _a_ L _l_, are red; if the screen be moved to the other side of _o_, the luminous circle will be bordered with violet, because it will be a section of the cone M _a_ M _l_, of which the exterior rays are violet. To avoid the influence of spherical aberration, and to render the phenomena of coloration more evident, let an opaque disc be placed over the central portion of the lens, so as to allow the rays only to pass which are at the edge of the glass; a violet image of the sun will then be seen at _v_, red at _r_, and, finally, images of all the colors of the spectrum in the intermediate space; consequently, the general image will not only be confused, but clothed with prismatic colors. To overcome the difficulty arising from the chromatic aberration, the optician has only to employ a combination of lenses of opposite focal length, and cut from glass possessing different refrangible powers, so that the rays of light passing through the one are strongly refracted, and in the other are bent asunder again, reproducing white light.