CH. IX] CORRECTION OF APOCHROMATIC OBJECTIVES 291 



gible from this how it is possible to -produce colorless images by com- 

 bining flint-glass concave and crown-glass convex lenses; or other 

 pairs of lenses where the dispersion and refraction give comparable 

 results. 



In making the color corrections for the lenses, the spherical cor- 

 rections were also made; the extent of both corrections attained up 

 to the present is discussed below. 



465. Corrections in Achromatic and Apochromatic objectives. - 

 (i) Spherical aberration. In achromatic objectives the spherical 



FIG. 173. ACHROMATIC COMBINATIONS OF CROWN AND FLINT GLASS LENSES. 

 (From Lewis Wright's Optical Projection). 



C C C C C C Thin edge or converging crown glass lenses. 



F F F F F F Thick edge or diverging flint glass lenses. The flint glass over- 

 comes the dispersion without overcoming the mean refraction, hence all these 

 combinations are converging. 



aberration is corrected for one color only; in apochromatic objectives 

 for two colors. (2) Chromatic aberration. In achromatic objectives 

 correction is made for two colors; in apochromats for three colors. 



In the apochromats it was found impossible to make the high 

 corrections necessary even with all the new glasses made available 

 by the Jena glass works; but with the new forms of glass and a natural 

 mineral, fluorspar, fluorite, calcium fluoride, with its very low index of 

 refraction and small dispersion, it was found possible to make the 

 fundamental advance in microscope objectives represented by the 

 apochromatic objectives. 



The possibility of bringing three colors to one focus makes the 

 apochromatic objectives especially valuable for photography. The 



