122 



Mica Wave-Plate. A thin sheet of mica for measuring the 

 retardation of polarised light. [2, 3, 22, 26.] 



Michelsen's Actinometer. See Actinometer. [77, 20, 21, 26.] 



Michelson's Interferometer. A special form of interferometer 

 (q.v.). 



Michelson's Pyrheliometer. See Pyrheliometer. 



Micro-Electroscope. A form of electroscope by Mr. C. T. R. 

 Wilson, in which a strip of gold leaf hangs beside a prolonga- 

 tion of a wire, to which it is attached at its upper end. 

 When the electroscope is charged the wire repels the gold 

 leaf, the deflection of which is measured either by a micro- 

 scope with micrometer eyepiece, or against the reflected 

 image of a millimetre scale. [4, 19.] 



Micrograph. A pantograph (q.v.) for executing extremely 

 minute writing and engraving. 



Micromanometer. A sensitive manometer (q.v.) for measuring 

 very small differences of pressure. [4, 13, 17.] 



Micromanometer, Chattock Tilting. A manometer in the form 

 of a modified V-tube water gauge. The principle of the 

 gauge is that of a V-tube in which the difference of the 

 pressures on the surfaces of the water in the two limbs of 

 the tube is measured by tilting the gauge through a very 

 small angle so that there is no displacement of the water 

 along the tube. [4.] 



Micromanometer, Pry's. An instrument in which the two 

 pressures whose difference is to be measured are applied to 

 the two sides of a stretched circular membrane. The resulting 

 displacement of the centre of the membrane twists a sus- 

 pended mirror, and the deflection of a spot of light reflected 

 from this mirror indicates the pressure difference. [4.] 



Micromanometer, Threlfall's. An instrument consisting essent- 

 ially of a V-tube containing oil or water, the two limbs of 

 which are formed by air-tight vessels. The difference in 

 pressure between the two limbs is determined by means of 

 a micrometer screw adjusted until its tip just touches the 

 surface of the liquid in one of the limbs, the micrometer 

 head being graduated to give readings of pressure difference 

 directly. [4, 21.] 



Micrometer. (i) An instrument used with a telescope or micro- 

 scope to measure small distances or the apparent diameters 

 of objects which subtend very small angles. (2) A mech- 

 anical device for measuring small lengths in engineering and 

 other work. [2, 3, 9, 13, 14, 17, 18, 20, 21, 22, 24, 25, 26, 27] 



Micrometer Balance. A balance for ascertaining minute differ- 

 ences in weight. [26.] 



Micrometer, Bifilar. A micrometer with two threads, one of 

 which at least is movable. [2, 3, 11, 17, 20, 22, 25, 26] 



Micrometer, Calliper. A calliper with a micrometer screw for 

 measuring dimensions with great accuracy. See also 

 Calliper. [2, 5, 19, 25] 



