Ramsdenn.
/ˈramzdən/, U.S. /ˈræmzd(ə)n/ Optics.
1. 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.
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the world > physical sensation > sight and vision > optical instruments > lens > [noun] > eye-piece
eyeglass1665
ocular1702
eyepiece1729
Ramsden1787
field glass1797
negative eyepiece1831
positive eyepiece1842
Kellner1865
orthoscopic1868
eye-point1875
comparison eyepiece1940
1787 Philos. Trans. (Royal Soc.) 77 55 The telescope..was an achromatic object-glass of Dollond, of 16 inches focal length, and 2 inches aperture, with a Ramsden's eye-glass, magnifying about 25 times.
1847 R. Potter Elem. Treat. Optics I. vii. 117 Ramsden's eyepiece is formed of a plano-convex and a convexo-plane lens of equal focal length.
1877 T. W. Webb in G. F. Chambers Handbk. Descr. Astron. (ed. 3) vii. viii. 745 The Ramsden ocular is never achromatic.
1900 R. A. Herman Treat Geom. Optics vi. 137 Both Huyghens' and Ramsden's eye-pieces have convergent powers, and the image appears inverted.
1972 R. Galopin & N. F. M. Henry Microsc. Study Opaque Minerals iii. 68/1 The Kellner type is a Ramsden ocular with an achromatised eye lens.
1988 R. E. Hopkins in D. Malacara Methods Exper. Physics XXV. iii. 62 The Ramsden eyepiece is used in narrow-field telescopes and microscopes when a cross hair is needed.
2. Ramsden's ghost n. now historical and rare an apparatus consisting of one or more convex lenses which projects the image of a contrasting object onto a plumb line, or projects the image of the plumb line onto a wire, in order to align a telescope which is mounted in a transit circle, mural circle, quadrant, or sector; (also) the image produced by this apparatus.
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the world > physical sensation > sight and vision > optical instruments > apparatus > [noun]
spectacle-case1597
steel glass1662
dark glasses1733
bruiser1738
hone pavement1738
Ramsden's ghost1807
sunshade1829
optical bank1874
phacometer1876
optical bench1880
flat1897
lens paper1925
light pipe1939
lens tissue1941
optical fibre1960
1793 F. Wollaston in Philos. Trans. (Royal Soc.) 83 139 I approve much of Mr. Ramsden's ghost, as it is called, where it can be used with safety.]
1807 A. Rees Cycl. (1819) VIII. at Circle This ingenious contrivance of producing an image in the open air has been denominated Ramsden's ghost by succeeding instrument-makers from the name of its inventor.
1819 A. Rees Cycl. XXIX. at Quadrant The mark used for each wire to bisect is a luminous disc, known by the name of Ramsden's ghost, from its being only the image of a luminous point without substance.
1851 Astron. Observ. National Observatory Washington 1846 II. p. x The Circle is adjusted to a vertical position by means of a plummet suspended on a fine silver wire, with the aid of appendages attached to the telescope known as ‘Ramsden's Ghosts’.
2016 A. McConnell Jesse Ramsden (new e-book ed.) vi. Primary levelling was done with the plumb-line, then adjusted with ‘Ramsden's ghost’.
3.
a. Ramsden circle n. the exit pupil of a telescope or microscope; the circle of least diameter of the bundle of rays leaving the eyepiece.
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?1883 Proc. 5th Ann. Meeting Amer. Soc. Microscopists 1882 128 If a piece of paper be held just above the eye-piece of a monocular compound microscope,..on the paper appears a circular spot of light, an image of the posterior clear aperture of the objective, which is known as the Ramsden circle.
1958 R. M. Allen Photomicrography (ed. 2) iv. 205 Where it is possible with binocular eyepieces (as with the Zeiss model) to employ half-moon diaphragms at the Ramsden circle, good stereoscopic effects can be obtained.
2002 S. F. Ray Appl. Photogr. Optics (ed. 3) liii. 449/1 The eye relief is only 4.5 to 15 mm, which may not allow the entrance pupil to coincide with the Ramsden circle.
b. Ramsden disc n. = Ramsden circle n. at sense 3a.
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1906 A. E. Wright Princ. Microsc. vii. 79 This is variously designated the Ramsden disc, [etc.].
1947 A. J. Thompson Making Your Own Telescope ix. 127 The diameter of the exit pupil can be quite exactly measured by means of calipers or a fine scale held in the plane of the Ramsden disk.
2003 N. James & G. North Observing Comets iii. 47 The eyepiece of a telescope produces a disk of light, usually known as the exit pupil, or the Ramsden disk,..which is, in effect, the image of the telescope objective formed by the eyepiece.
This entry has been updated (OED Third Edition, June 2008; most recently modified version published online March 2022).
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n.1787