What converts for curvature in a field so that a flat field with uniform focus is seen?
a. Achromatic objective lens
b. Plan-achromatic objective lens
c. Spherical correction ocular lens
d. Blue filter when placed above the condenser
ANS: B
A plan-achromatic objective lens, though more expensive, provides a flat field so that uniform focus is seen across a slide. An achromatic objective lens does not correct for the curvature in a round field, and the edges of the field appear fuzzy or out of focus when the center is in focus. No such thing as a spherical ocular lens exists; a blue filter eliminates the yellow emitted by a tungsten-halogen light bulb.
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