_The Compensating Eye-piece._--The very important improvements effected in the construction of the objective naturally led up to an equally useful change for the better in the eye-piece.
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.
Fig. 105.--A sectional view of Zeiss’s Compensating series of Eye-pieces, 1/2 the full size.
B.--Lower focal plane of eye-pieces, with their lenses _in situ_.]
The ordinary working eye-pieces, Huyghenian and others, commencing with a magnification of four diameters, are so constructed that they can be conveniently used, as we are accustomed to use them in England, with high powers, Zeiss’s Nos. 12 and 18 compensating eye-pieces being adapted for use with his lower power apochromatic lenses of 16 and 8 mm. The numbering of the eye-pieces is carried out on the plan originally proposed by Professor Abbe--that is, the number denotes how many times an eye-piece, when employed with a given tube-length, increases the initial magnifying power of the objective, and at the same time furnishes figures for their rational enumeration. It is on this basis that the German compensating eye-pieces have been arranged in series, and in agreement with their magnifying power and distinctive numberings of 2, 4, 6, 8, 12, 18. Of these several eye-pieces, 12 is found to be the most useful. The magnification obtained by combining a compensating eye-piece with any apochromatic objective is found by multiplying its number by the initial magnification of the objective, as given in the following proof:--An objective of 3·0 mm. focus, for example, gives in itself a magnification of 83·3 (calculated, for the conventional distance of vision, 250 mm.); eye-piece 12 therefore gives with this objective a magnification of 12 × 83·3 = 1000 diameters. The classification, however, of these eye-pieces, as furnished by Abbe, is dependent upon increase in the total magnifying power of the microscope obtained by means of the eye-piece as compared with that given by the objective alone. The numbering, then, denotes how many times an eye-piece increases the magnifying power of the objective when used with a given body-tube; the proper measure of the eye-piece magnification; and, at the same time, the figures for rational enumeration.