Page:Elementary Text-book of Physics (Anthony, 1897).djvu/455

§359] the correction of chromatic aberration in the object-glasses of telescopes.

Thus far nothing has been said of the relative separation of the different colors of the spectrum by refraction by different substances. Suppose two prisms of different substances to have such refracting angles that the spectra produced are of the same length. If these two spectra be superposed, the extreme colors may be made to coincide, but the intermediate colors do not coincide at the same time for any two substances of which lenses can be made. Perfect achromatism by means of lenses of two substances is therefore impossible. In practice it is usual to construct an achromatic combination to superpose, not the extreme colors, but those that have most to do with the brilliancy of the image.

The indistinctness due to chromatic aberration, existing even in the compound objective, may be much diminished by a proper disposition of the lenses of the eyepiece. Fig. 134 shows the negative or Huygens eyepiece.

Let $$A$$ be the objective of a telescope or microscope. A point situated on the secondary axis $$ov$$ would, if the objective were a single lens, have images on that axis, the violet nearest and the red farthest from the lens. If the lens could be perfectly corrected, these images would all concide. By making the lens a little over-corrected, the violet may be made to fall beyond the red. Suppose $$r$$ and $$v$$ to be the images. $$B$$ and $$C$$ are the two lenses of the Huygens eyepiece. $$B$$ is called the field-lens, and is three times the focal length of $$C.$$ It is placed between the objective and its focal plane, and therefore prevents the formation of the images $$rv,$$ but will form images at $$r'v'$$ on the secondary axes $$o'r, o'v.$$ If everything is properly proportioned, $$r'v'$$ will fall on