in connexion with the achievement of a critical microscopic image that in the case of objectives designed to work respectively with the long, 250 mm. "English," or short, 160 mm. "Continental" tube, the critical image is obtained only when the tube length is adjusted exactly to these standard focal lengths.
(2) A lens system may be designedly over- or under-corrected by the maker.
We shall see hereafter (Cap. XIII, subsect. 35, infra) that dry objectives are habitually under-corrected in order to make provision for the distortion of the opening limb of the beam, which is effected by refraction where the beam emerges from the cover-glass into air.
The curvature of the field, which has been enumerated among the distortion effects produced by spherical aberration, is not abolished by the corrections which are introduced for the purpose of rendering the lens aplanatic.
Fig. 40.
SHOWING THE NATURE OF THE DISTORTION WHICH IS ASSOCIATED WITH THE OVER-CORRECTION OF THE SPHERICAL ABERRATION OF A LENS.
4. Distortion due to chromatic aberration.
In view of their different refrangibility, the different components of white light are in the case where they traverse an uncorrected lens, brought to focus on different focal planes. In particular, the more refrangible blue rays are brought to focus on a nearer, the less refrangible red light on a plane which is more remote from the lens (Plate VII, Fig. 1).
The effect of this chromatic aberration upon the image is two-fold—
(1) The image of every radiant point is encompassed by coloured fringes corresponding to the diffusion discs of those components of the beam which focus, as the case may be, upon a nearer or a farther focal plane than that which happens for the moment to be under examination.
