Ch. XIV] LENSES AND THEIR ACTION 583 § 810. Chromatic aberration.—By this is mount the separation of the images produced by the different wave lengths of which white light is composed. Newton thought this was a purely refractive action and therefore could not be corrected without at j the same time overcoming all the refraction, hence he thought there could be no images formed by lenses or combinations of lenses without the presence of the color defect. But later it was found that some glass separated the light into colors more markedly than others of the same refraction. Now by combining two kinds of glass which act differently in this respect it was found possible Fig. 324. Achromatic Lexsks. (From Lacis Wright, Optical Projection). By combining a convergent or convex crown glass lens with a divergent <*r concave flint glass lens it is possible to get a combination which is largely tree from chromatic as well as spherical aberration. In all but D and the right- hand combination, but two lenses are used; in those, one flint and two crown glass lenses are used. to bring two or three of the colors to one focus, and thus to produce practically colorless images by means of lenses (fig. 324). Usually an objective for forming images- photographic objec¬ tive, microscopic objective, projection objective—is corrected l with for spherical and for chromatic aberration, so that the image is correct in every way. This is accomplished bv combining concave and convex lenses of the right form and composition. Sometimes also, as with the apochromatic, microscojK? objectives, a natural mineral—fluorite—is introduced to make a more ixrfect correc¬ tion than could be accomplished by artificial glass.