10 THE MICROSCOPE AND ITS REVELATIONS. 



its marginal portion shall exhibit broad fringes, as is well seen in the pic- 

 tures exhibited by non-achromatic Oxhydrogen-Microscopes. 



13. Although 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, 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 con- 

 struction 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 colors. No such correc- 

 tion 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 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 con- 

 vex 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 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 jtfm^-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 7f inches, will 

 produce the same degree of color as a convex lens of crown-glass whose 

 focal length is 4J inches. Hence a concave lens of the former material and 

 curvature will fully correct the dispersion of a convex lens of the latter; 

 whilst it diminishes its refractive power to such an extent only as to make 

 its focus 10 inches. A perfect correction for Chromatic Aberration might 

 thus be obtained, 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, especially towards the margin of the 

 field, being bordered on one side with a purple fringe, and on the other 

 with a green. In the best constructed combinations, however, whether 

 for the Telescope or the Microscope, the chromatic error is scarcely 

 perceptible; the aberrations of the objective being so arranged as to be 

 .almost entirely compensated by the opposite aberrations of the eye-piece 

 ( 27). 



14. It was in the Telescope that the principle of correction for Chro- 

 matic dispersion, which had been theoretically devised by Euler and 

 other mathematicians, was first carried into practical application; an 

 Achromatic object-glass having been constructed in 1733 by Hall, and a 

 more perfect combination having been ' worked out in 1757 by Dollond, 

 whose system, known as "the ' telescopic triplet/ remains in use to the 

 present time. This triplet consists of a double-concave lens of flint-glass, 

 interposed between two double-convex lenses of crown; such curves being 

 given to their respective surfaces, as serve almost entirely to extinguish 

 not only the Chromatic, but the Spherical aberration, in the case of rays 

 proceding from distant objects, which fall on the surface of the object- 

 glass in a direction that is virtually parallel. These rays form an image 

 in the ' principal focus ' of the object-glass, the size of which varies with 

 its distance from the lens; magnifying power being thus gained by 



