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What does the inscription 0.17 on the objective signify?

April 26, 2017inscription Objective

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What does the inscription 0.17 on the objective signify?

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0.17 refers to the thickness, in millimeters, of the cover glass that was assumed by the lens designer in computing the corrections for the objective. For objectives with a numerical aperture higher than 0.45, a departure in this assumed thickness (or no cover glass at all) may result in deterioration of the image.