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Learn about microscope aberrations.
Microscope Aberrations
Two factors arise which prevent the advantageous use of more than about 25 diameters in magnifiers ; they are called the chromatic and spherical aberrations. The first is the term employed when the object is apparently fringed with color, predominently blue and yellow ; the second, when all but the central portion of the lens shows the object indistinctly ; these faults increase with the magnifying power. In the case of a combination of several lenses, they may partially be overcome by interposing an opaque plate with a small opening, called a diaphragm, between them, which cuts off the outer or marginal rays, or the lenses may be made of a smaller diameter. An incision may also be cut into the glass equally between the two surfaces, when from the name of the inventor, it is called a Coddington. The most approved method, however, for eliminating these appearances, is by the use of one or two con cave flint glass lenses in connection with the double convex crown glass lens. When the color or chromatic aberration is thus removed, the lenses are said to be achromatic, and when both the chromatic and spherical aberrations are avoided, the lens is called aplanatic, and is then said to be corrected. An achromatic lens, composed of one flint and one crown glass lens, is called a doublet ; one with two flint glass lenses and one crown glass called a triplet. The latter is the best form, as it gives the highest correction ; such a lens (it is thus called from the the fact that the lenses are cemented together and act like one) may be held with either side toward the object with equally good results. At quite an obliquity, without loss of definition ; this feature is important, as it is almost impossible to give a lens a theoretically correct position to both the eye and object with the unaided hand.
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