This is effected by combining, as shown in fig. 4, a convex lens of
_crown_ glass with a concave lens of _flint_ glass, the convex lens
being placed nearest to the object. A little colour still remains, but
not enough to interfere seriously with the distinctness of the image.

But even if the image formed by the object-glass were perfect, yet this
image, viewed through a single convex lens of short focus placed as in
fig. 1, would appear curved, indistinct, coloured, and also _distorted_,
because viewed by pencils of light which do not pass through the centre
of the eye-glass. These effects can be diminished (but not entirely
removed _together_) by using an _eye-piece_ consisting of two lenses
instead of a single eye-glass. The two forms of eye-piece most commonly
employed are exhibited in figs. 5 and 6. Fig. 5 is Huyghens' eye-piece,
called also the _negative_ eye-piece, because a real image is formed
_behind_ the _field-glass_ (the lens which lies nearest to the
object-glass). Fig. 6 represents Ramsden's eye-piece, called also the
_positive_ eye-piece, because the real image formed by the object-glass
lies _in front of_ the field-glass.

[Illustration: _Fig. 5._]

[Illustration: _Fig. 6._]

The course of a slightly oblique pencil through either eye-piece is
exhibited in the figures. The lenses are usually plano-convex, the
convexities being turned towards the object-glass in the negative
eye-piece, and towards each other in the positive eye-piece. Coddington
has shown, however, that the best forms for the lenses of the negative
eye-piece are those shown in fig. 5.