82 



Dines's Self-Recording Mercurial Barometer. See Barometer. 

 [2, 13, 15, 17, 20, 21, 26.] 



Diopter. An old form of theodolite (q.v.). [27.] 



Dioptric Light. A plan of lighting used in lighthouses in which the 

 optical system is one of refraction instead of reflection. [7.] 



Dioptric Micrometer. See. Micrometer. [25.] 



Dioptric Telescope. See Telescope. [1, 3, 11, 13, 17, 20.] 



Dip Circle. An instrument for determining the magnetic dip 

 at any spot on the earth's surface. [2, 4, 6, 13, 15, 17, 19, 

 21, 25, 27.] 



Dip Circle, Fox. [2, 17.] < 



Dip Compass. A magnetic needle suspended between centres 

 so as to move readily in a vertical plane. [2, 6, 13, 15, 16, 

 17, 21, 24, 25, 27.] 



Dip Inductor. An instrument for determining the direction of 

 a magnetic field, depending on the principle that when a 

 coil of wire is rotated in the field using a suitable commu- 

 tator the resulting E.M.F. only vanishes when the axis of 

 rotation is parallel to the direction of the magnetic field. [4.] 



Dip Needle. A magnetic needle, suspended at its centre of 

 gravity and moving in a vertical plane so as to indicate on 

 a graduated circle the magnetic dip or inclination. [2, 6, 

 13, 15, 16, 17, 18, 21, 24, 27, 28.] 



Dip Sector. An astronomical reflecting instrument. [27.] 



Dipleidoscope. An instrument for determining the apparent 

 time of noon. It consists of two mirrors and a plane glass 

 disposed in the form of a prism, so that by the reflection 

 of the sun's rays from their surfaces, two images are pre- 

 sented to the eye, moving in opposite directions and co- 

 inciding at the instant when the sun's centre is on the 

 meridian. [2, 26.] 



Dipping Refractometer. See Refractometer. [14, 26.] 



Director. A form of military theodolite (q.v.) for directing 

 gunfire. [3, 8, 13, 18, 20, 21, 24, 27.] 



Direct-Vision Liquid Prism. See Prism. [2, 3, 14, 20, 26.] 



Direct-Vision Spectroscope. See Spectroscope. 



Discharging Rod. An instrument to discharge a charged 

 electrical jar or battery. It has a glass handle and a pair 

 of hinged rods with baUs on the ends, which are brought 

 into connection respectively with the two surfaces or poles 

 of the jar or battery. [26.] 



Dissecting Microscope. A microscope with rack adjustment for 

 focus, spring clips to hold the object-slide, movable arm 

 for carrying the lenses, usually of low magnification, for 

 examining anatomical and botanical objects. [2, 3, 5, 9, 

 17, 20, 21, 22, 26] 



Distance-Finder. A rangefinder. [3, 13, 15, 17, 20, 21, 27, 28.] 



Distance-Finder, Alidade (Reeves's). A form of alidade provided 

 with two small telescopes which can be attached to the ends 

 of the rule and used for range-finding. [28.] 



