4._] 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.