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 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._]