. The microscope : an introduction to microscopic methods and to histology. Microscopes. CH. I] MICROSCOPE AND ACCESSORIES 5 \ 10. Correction of Chromatic and of Spherical Aberration.âEvery simple lens has the defect of both chromatic and spherical aberration, and to overcome this, kinds of glass of different refractive power and different dis- persive power are combined, concave lenses neutralizing the defects of convex lenses. If the concave lens is not sufficiently strong to neutralize the aberra- Fig. 13. The ray (o) near the edge of the lens is brought to a focus nearer the lens than the
. The microscope : an introduction to microscopic methods and to histology. Microscopes. CH. I] MICROSCOPE AND ACCESSORIES 5 \ 10. Correction of Chromatic and of Spherical Aberration.âEvery simple lens has the defect of both chromatic and spherical aberration, and to overcome this, kinds of glass of different refractive power and different dis- persive power are combined, concave lenses neutralizing the defects of convex lenses. If the concave lens is not sufficiently strong to neutralize the aberra- Fig. 13. The ray (o) near the edge of the lens is brought to a focus nearer the lens than the rav (i). Both are brought to a focus sooner than 'rays very near the axis. (/) Principal focus for rays very near the axis ; (/')â Focus for the ray (i), and (f") Focus for the ray (0). Intermediate rays Fig. 13. Double Convex Lens, showing would cross the axis all the Spherical Aberration, way from (f tof). tions of the convex lens, the combination is said to be under-corrected, while if it is too strong and brings the marginal rays or the blue rays to a focus beyond the true principal focus, the combination is over-corrected. In Newton's time there was supposed to be a direct proportion between the refractive power of any transparent medium and its dispersive power (i. e. its power to separate the light into colors). If this were true then the con- tention of Newton that it would' be impossible to do away with the color without at the same time doing away with the refraction would be true and useful achromatic combinations would be impossible. It was found by experi- ment, however, that there is not a direct ratio between the refractive and dispersive powers for the different colors in different forms of glass, so that it is possible to do away largely with chromatic aberration and retain sufficient refraction to make the combination serve for the production of images. (See also the discussion under apochromatic objectives \ 25.) Probably no higher technical skill is u
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