112 



Lantern, Fresnel. An optical projection lantern having its 

 lamp or light surrounded by a hollow cylindrical Fresnel 

 lens (q.v.). [26. ~\ 



Laryngoscope. An instrument consisting of two mirrors for 

 reflecting light upon and examining the larynx. [2, 13, 26.] 



Laryngostroboscopic Apparatus. A method 'for observing the 

 vibration of the vocal chords during the production of 

 sounds. It consists of a laryngoscope, and a means of 

 rapidly interrupting the source of light so that the chord 

 may be observed at any periodic position. 



La Trobe Beam Compass. See Beam Compass. [13, 21.] 



Lea Biram Anemometer. See Anemometer. [13, 21.] 



Leach and Lythgoe's Refractometer Scale. See Refractometer 

 Scale. 



Lean's Dial. An old form of mining dial (q.v.) invented by 

 Mr. J. Lean. [6, 13, 17, 21, 24, 25.] 



Lean's Miner's Theodolite. See Theodolite. [6, 13, 18, 21, 25.] 



Lecture Galvanometer. A type of galvanometer (q.v.) with 

 large mirror for projection ; adapted to demonstration 

 purposes. [4, 10] 



Leeway Indicator. [15.] 



Leffmann Beam Centrifugal Machine. For milk analysis. See 

 Centrifuge. 



Lencoscope. An instrument for testing the colour perception 

 of the eye or for comparing different lights as to their 

 constituent colours or their relative whiteness. 



Lens. In optics, a portion of a refracting medium bounded by 

 two curved surfaces, or by one plane surface and one curved 

 surface. Usually made of glass or other transparent sub- 

 stance. See also Polyoptrum and Polyscope. [1, 2, 3, 8, 9, 

 10, 11, 12, 13, 14, 15, 17, 18, 20, 21, 22, 23, 24, 26, 27] 



Lens, Achromatic. A lens corrected for chromatic aberration ; 

 as compared with an apochromatic lens (q.v.) it is corrected 

 for only two colours. 



Lens, Aplanatic. A lens which is free from spherical aberration 

 and satisfies the sine condition. [1, 2, 3, 8, 9, 10, 17, 20, 

 21, 22, 23, 26, 27] 



Lens, Apochromatic. A lens corrected for chromatic aberration 

 for the secondary spectrum. 



Lens, Coddington. A cylindrical lens with a deep groove cut in 

 midway between the ends and acting as a diaphragm giving 

 a good definition and wide field ; generally used as a pocket 

 magnifier. [2, 3, 8, 9, 13, 15, 17, 20, 21, 22, 24, 26, 27] 



Lens, Fluid. A lens in which a liquid is imprisoned between 

 circular glass discs of the required curvature. 



Lens, Focostat. A lens with slide and universal joint, but 

 attached to a dissecting needle ; for biological work. [3, 

 20, 26] 



