James Gregory, in his Optica Promota (1663), discusses the forms of images and objects produced by lenses and mirrors, and shows that when the surfaces of the lenses or mirrors are portions of spheres the images are curves concave towards the objective, but if the curves of the surfaces are conic sections the spherical aberration is corrected.
He did not attempt the formation of a parabolic figure on account of the probable mechanical difficulties, and he had besides satisfied himself that the chromatic and not the spherical aberration formed the chief faults of previous telescopes.
It requires the middle of the aperture stop to be reproduced in the centres of the entrance and exit pupils without spherical aberration.
Of thin positive lenses with n= 1-5, four are necessary to correct spherical aberration of the third order.
In a plane containing the image point of one colour, another colour produces a disk of confusion; this is similar to the confusion caused by two " zones " in spherical aberration.
