certain form we
produce a spectrum of the sun, this spectrum will be thrown a certain
distance away from the point on which the sun's rays would fall if not
interfered with. This distance depends on the _refractive_ power of the
glass. The spectrum will have a certain length, depending on the
_dispersive_ power of the glass. Now, if we change our prism for another
of exactly the same shape, but made of a different kind of glass, we
shall find the spectrum thrown to a different spot. If it appeared that
the length of the new spectrum was increased or diminished in exactly
the same proportion as its distance from the line of the sun's direct
light, it would have been hopeless to attempt to remedy chromatic
aberration. Newton took it for granted that this was so. But the
experiments of Hall and the Dollonds showed that there is no such strict
proportionality between the dispersive and refractive powers of
different kinds of glass. It accordingly becomes possible to correct the
chromatic aberration of one glass by superadding that of another.

[Illustration: _Fig. 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. 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