All objectives of wide aperture, from the curvature of their hemispherical front lenses, show a certain amount of colour defect in the extra-axial portion of the field, even if perfectly achromatic in the centre. Whether an image be directly projected by the objective, or whether it be examined with an aplanatic eye-piece, colour fringes may be detected, possibly in an increasing degree towards the periphery. This residual chromatic aberration has at length been very nearly eliminated by the aid of the compensating eye-piece. The construction of compensating eye-pieces is somewhat remarkable, since they have an equivalent error in an opposite direction--that is, the image formed by the red rays is greater than that corresponding to the blue rays; consequently, eye-pieces so constructed serve to compensate for the unequal magnification produced by different coloured rays, and images appear free from colour up to the margin of the field. Zeiss’s compensating eye-pieces are so arranged that the lower focal points of each series lie in the same plane when inserted in the body-tube of the microscope; no alteration of focus is therefore required on changing one eye-piece for another. This of itself is not only an advantage but also a saving of time, while the distance between the upper focal point of the objective and the lower one of the eye-piece, which is the determining element of magnification, remains constant. [Illustration: Fig. 105.--A sectional view of Zeiss’s Compensating series of Eye-pieces, 1/2 the full size. A.--Plane of the upper edge of the tube. B.--Lower focal plane of eye-pieces, with their lenses _in situ_.]