CHROMATIC ABBERATION. 73 



predominantly red tint, and will be surrounded by a series of 

 colored fringes in inverted order, formed by the other rays of the 

 spectrum, which have met and crossed.' The line e e, which 

 joins the points of intersection between the red and the violet 

 rays, marks the "mean focus," that is the situation in which the 

 colored fringes will be narrowest, the " dispersion" of the colored 

 rays being the least ; whilst the interval c d, which separates the 

 foci of the extreme rays, is termed the Chromatic Aberration of 

 the lens. As the axial ray a' b' undergoes no refraction, neither 

 does it sustain any dispersion ; and the nearer the rays are to 

 the axial ray, the less dispersion do they suffer. Again, the 

 more oblique the direction of the rays, whether they pass through 

 the central or the peripheral portion of the lens, the greater will 

 be the refraction they undergo, and the greater also will be their 

 dispersion ; and thus it happens that when, by using only the 

 central part of a lens (§ 12), the chromatic aberration is reduced 

 to its minimum, the central part of a picture may be tolerably 

 free from false colors, whilst its marginal portion shall exhibit 

 broad fringes.^ 



12. The Chromatic Aberration of a lens, like the Spherical, 

 may be diminished by the contraction of its aperture, so that 

 only its central portion is employed. But the error cannot be 

 got rid of entirely by any such reduction, which, for the reasons 

 already mentioned, is in itself extremely undesirable. Hence it 

 is of the "first importance in the construction of a really efficient 

 Microscope, that the chromatic aberration of its "object-glasses" 

 (in which the principal dispersion is liable to occur) should be 

 entirely corrected, so that the largest possible aperture should be 

 given to these lenses, without the production of any false colors. 

 S'o such correction can be accomplished even theoretically in a 

 single lens ; but it may be effected by the combination of two or 

 more, advantage being taken of the different relations which the 

 refractive and the dispersive powers bear to each other in different 

 substances. For if we can unite with a convex lens, whose dis- 

 persive power is low as compared with its refractive power, a 

 concave of lower curvature, whose dispersive power is relatively 

 high, it is obvious that the dispersion of the rays occasioned by 

 the convex lens may be effectually neutralized by the opposite 

 dispersion of the concave (§ 6) ; whilst the refracting power of 

 the convex is only lowered by the opposite refraction of the con- 

 cave, in virtue of the longer focus of the latter. ISTo difficulty 

 stands in the way of carrying this theoretical correction into 

 practice. For the "dispersive" power of ^^jw^glass bears so 

 much larger a ratio to its refractive power than does that of 



' This experiment is best tried with a lens of long focus, of which tlie central part is 

 covered with an opaque stop, so that the light passes only through a peripheral ring; 

 since, if its whole aperture be in use, the regular formation of the fringes is interfered 

 with by the spherical aberration, which gives a diiferent focus to the rays passing through 

 each annular zone. , j 



^ This is well seen in the large pictures exhibited by Oxy-hydrogen Microscopes. 



