CHROMATIC ABERRATION. 13 



rays being the least. As the axial ray a' b' undergoes no refrac- 

 tion, 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 toler- 

 ably free from false colours, 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 a large aperture may be given to these lenses without the 

 production of any false colours. No such correction can be accom- 

 plished 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 dispersive power is low as compared to 

 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 Refract- 

 ing power of the convex is only lowered by the opposite refraction 

 of the concave, in virtue of the longer focus of the latter. — No 

 difficulty stands in the way of carrying this theoretical correction 

 into practice. For the ' dispersive ' power of flint-glass bears so 

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

 glass, that a convex lens of the former whose focal length is 7| 

 inches, will produce the same degree of colour as a convex lens of 

 crown-glass whose focal length is 4| inches. Hence a concave lens 

 of the former material and curvature will fully correct the disper- 

 sion of a convex lens of the latter ; whilst it diminishes its refrac- 

 tive power to such an extent only as to make its focus 10 inches. 

 The correction for Chromatic Aberration in such a lens would be 

 perfect, if it were not that although the extreme rays — violet and 

 red — are thus brought to the same focus, the dispersion of the rest 

 is not equally compensated ; so that what is termed a secondary 

 spectrum is produced, the images of objects seen through such a 



* This is well seen in the large pictures exhibited by Oxy-hydrogen 

 Microscopes. 



