Attributive and (now rare) in the genitive. Designating an eyepiece consisting of two planoconvex lenses of equal focal length with their convex surfaces towards each other and a separation less than their focal length.
This arrangement has the focal plane outside the eyepiece, and so lends itself to use with a graticule or cross hairs.
Late 18th century; earliest use found in Philosophical Transactions of the Royal Society of London. From the name of Jesse Ramsden, English instrument-maker, who described the eyepiece in 1783 (Philos. Trans (Royal Soc.) 73 94).