Apochromat

Apochromat (from Greek apo- meaning away and chroma meaning color), microscope objectives in which the greatest elimination of spherical and chromatic aberration has been achieved. This is accomplished by combining different types of glass (crown glass and flint glass) with fluorite (fluorspar). In an apochromat, three different rays are brought to a single point on the optical axis, which almost completely eliminates chromatic aberration; spherical aberration is also eliminated for two different rays. Thanks to this, an apochromat produces a completely uncolored image for the eye. However, rays from the right part of the spectrum give a larger image than rays from the left part, and a cross-section of an apochromat shows that it is necessary to combine with so-called compensating eyepieces, which give a larger image for the left part of the spectrum; this achieves the bringing together of different images. In addition to their high cost, apochromats have another disadvantage: the image they produce appears convex, and if the center of the field of view is precisely adjusted, the edges will appear blurry, which is a significant inconvenience when photographing with high magnifications. However, for ordinary microscopic examination, this disadvantage, which can be easily corrected by adjusting the micrometer screw, is more than compensated for by the extraordinary clarity of the image both in terms of contours and color. Furthermore, apochromats have a larger aperture (see) compared to achromats and therefore provide more fine structural details (see Microscope).