123 



Micrometer Circular Ring. A metallic ring fixed in the focus 

 of the object-glass of a telescope and used to determine 

 differences of ascension and declination between stars. 



Micrometer, Cobweb. A micrometer in which cobweb threads 

 are used. Usually consists of a viewing eyepiece, cobwebs 

 mounted on a slide, and a micrometer screw with divided 

 scale. [2, 3, 17, 21, 22, 26, 27.] 



Micrometer, Crack. A micrometer for detecting and measuring 

 the relative movements of the two sides of a crack or fissure 

 in a building and so ascertaining how the crack is developing. 



w 



Micrometer Dial Calliper. A calliper with micrometer screw, 

 and dial from which the dimensions are read. See also 

 Calliper. 



Micrometer, Dioptric. A form of double image micrometer, 

 introduced by Ramsden, in which the divided lens is in the 

 eyetube. [25.] 



Micrometer, Double Image. A micrometer in which two images 

 of the object are formed in the field of an optical instrument, 

 usually by the two halves of a bisected lens which are 

 movable along their line of sector by a screw, and distances 

 are determined by the number of screw revolutions necessary 

 to bring about optical coincidence. [3, 13, 25.] 



Micrometer, Double Refraction. A double image micrometer 

 in which the two images are formed by the double refraction 

 of rock or crystal or Iceland spar. [3, 13, 25.] 



Micrometer Eyepiece. See Eyepiece, Micrometer. [2, 3, 10, 18, 

 19, 20, 21, 27.] 



Micrometer, Filar. Another name for the cobweb micrometer 

 (q.v.). [2, 3, 4, 17, 18, 20, 22, 27.] 



Micrometer, Gauge. See Micrometer Calliper. [19.] 



Micrometer, Linear. A minute and very delicately graduated 

 scale of equal parts. [17.] 



Micrometer Microscope. A compound microscope combined 

 with a filar micrometer, used chiefly for subdividing the 

 divisions of astronomical and geodetical instruments, and 

 for exact measurements of small distances. [2, 3, 17, 21, 22, 

 25, 26.] 



Micrometer, Position. A micrometer applied to the tube of an 

 astronomical telescope for measuring angles of position in 

 the field of view. 



Micrometer, Reeves's Tangent. A tangent micrometer screw for 

 use on theodolites and sextants. [6.] 



Micrometer, Reticulated. A form of micrometer for measuring 

 small celestial distances. 



Micrometer Screw. A screw fitted to optical and mathematical 

 instruments as a means for exact measurement of very 

 small angles or distances. [2, 3, 15, 17, 20, 21, 22, 24, 25, 

 27.] 



