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.