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What is the purpose of the correction collar found on some “dry” 20x, 40x, or 60x objectives?

April 26, 2017collar dry purpose

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What is the purpose of the correction collar found on some “dry” 20x, 40x, or 60x objectives?

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The correction collar, when rotated, separates or brings together some of the internal glass elements of the objective. This action can correct for incorrect cover glass thickness. In upright microscopes, the range of correction of the collar is usually from 0.11-0.22 millimeters. For inverted microscopes, the range of correction capability is from 0 (uncovered) to 2 millimeters to correct for thick culture vessels. The purpose is to eliminate spherical aberration.