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Ramsden eyepiece

Ramsden eyepiece

(Science: microscopy) An ocular consisting of two plano-convex lenses with the plane side of the lower lens nearer the objective. The focal plane is outside the system, hence it is of the positive type with the diaphragm below the lenses. While this eyepiece gives a somewhat flatter field than the Huygenian and for this reason has been used in the past for micrometre oculars, the colour correction is poor with the formation of colour fringes around the object. The modern ramsden eyepiece has an achromatic doublet for the eye lens to correct for colour.


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