CH. XIV] LENSES AND THEIR ACTION 583 



810. Chromatic aberration. By this is meant 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 ] 

 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 LENSES. 

 (From Lewis Wright, Optical Projection). 



By combining a convergent or convex crown glass lens with a divergent or 

 concave flint glass lens it is possible to get a combination which is largely free 

 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 both 

 for spherical and for chromatic aberration, so that the image is 

 correct in every way. This is accomplished by combining concave 

 and convex lenses of the right form and composition. Sometimes 

 also, as with the apochromatic, microscope objectives, a natural 

 mineral fluorite is introduced to make a more perfect correc- 

 tion than could be accomplished by artificial glass. 



