an object-glass or object-mirror differs from
a real object.

The peculiarities to be noted are the _curvature_, _indistinctness_, and
_false colouring_ of the image.

The curvature of the image is the least important of the three defects
named--a fortunate circumstance, since this defect admits neither of
remedy nor modification. The image of a distant object, instead of lying
in a plane, that is, forming what is technically called a _flat field_,
forms part of a spherical surface whose centre is at the centre of the
object-glass. Hence the centre of the field of view is somewhat nearer
to the eye than are the outer parts of the field. The amount of
curvature clearly depends on the extent of the field of view, and
therefore is not great in powerful telescopes. Thus, if we suppose that
the angular extent of the field is about half a degree (a large or
low-power field), the centre is nearer than the boundary of the field by
about 1-320th part only of the field's diameter.

The indistinctness of the image is partly due to the obliquity of the
pencils which form parts of the image, and partly to what is termed
_spherical aberration_. The first cause cannot be modified by the
optician's skill, and is not important when the field of view is small.
Spherical aberration causes those parts of a pencil which fall near the
boundary of a convex lens to converge to a nearer (_i.e._ shorter) focus
than those which fall near the centre. This may be corrected by a proper
selection of the forms of the two lenses which replace, in all modern
telescopes, the single lens hitherto considered.

The false colouring of the image is due to _chromatic aberration_. The
pencil of light proceeding from a point, converges, not to one point,
but to a short line of varying colour. Thus a series of coloured images
is formed, at different distances from the object-glass. So that, if a
screen were placed to receive the mean image _in focus_, a coloured
fringe due to the other images (_out of focus, and therefore too large_)
would surround the mean image.

Newton supposed that it was impossible to get rid of this defect, and
therefore turned his attention to the construction of reflectors. But
the discovery that the _dispersive_ powers of different glasses are not
proportional to their reflective powers, supplied opticians with the
means of remedying the defect. Let us clearly understand what is the
discovery referred to. If with a glass prism of a