David,
I will give it a try, though it is easier in pictures then words. A
prism disperses light into different colors because the index of refraction
varies with the wavelength of light passing trough it (i.e. the dispersion
of the glass). If you look at the edge of a lens it looks like a prism and
disperses light accordingly. Now Newton noticed that if you take two prisms
and place them so that they disperse in opposite directions you can
recombine the original white light into white light again (but if you remove
a colored portion say by inserting a card between the prisms, the light
recombines to the complementary color of that which is removed. Thus he
concluded that colored light is more fundamental than white light.
If we now return to our lens, by placing a complementary lens (one
with a negative focal length that is the same a positive lens, and made of
the same glass) the colors will be removed, but so will the converging
properties of the lens. What make the construction of an achromatic possible
is that the dispersion of glass is not a simple function of the index of
refraction of the glass, so we can replace our negative lens above with a
weaker (longer focal length), but more dispersive glass lens that will still
recombine the colors, but the combination will have a net positive focal
length. This combination is an achromatic lens.
Since the dispersion is not a simple function of the wavelength, it
is only possible to get color correction for two colors (usually adjusted
for the hydrogen red and blue lines). With more glass it is possible to
correct for three wavelengths. Lenses that are so corrected are called
apochromatic).
As an historical aside, Newton thought of the achromat but measured
the dispersion of two glass and concluded that the dispersion was
proportional to the index of refraction, and a achromat was not possible, so
he invented the reflecting telescope. English opticians not knowing of
Newton's conclusion, started making achromats about fifty years later.
I hope this doesn't further muddy the waters for you.
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU