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 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.