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.