128 



that the ordinary beam of light produced by double re- 

 fraction is thrown out of the field by total reflection from 

 the internal cemented surface, and a polarised beam of 

 light alone is transmitted. See also Prism. [2, 3, 20, 22, 

 25, 26.} 



Night Glass. A Galilean binocular (q.v.) with large object-glass 

 giving great illumination at night. [15]. 



Night Surveying Lamp. A lamp used in mine surveying. [6, 15, 

 27.] 



Nilometer. An instrument for measuring the rise of water in 

 the Nile during its periodic flood. 



Niloscope. A Nilometer. 



Nitrogen Apparatus. For the estimation of nitrogen. 



Nitrometer. An apparatus for determining the amount of 

 nitrogen or some of its compounds in any substance 

 subjected to analysis. An azotometer. 



Noctograph. An instrument or register which records the 

 presence of watchmen on their beats. 



Nocturnal or Nocturlabrium. An old form of instrument, chiefly 

 used at sea, to take the altitude or depression of the pole 

 star, for finding the latitude and the hour of the night. 



Normatachograph. An instrument for determining and regis- 

 tering the duration of more or less complex operations of 

 the mind. 



Norrenberg's Polariscope. See Polariscope. [3, 26.] 



Nose-piece. The end of a microscope body to which the object- 

 glass is attached. Double, triple, or quadruple nosepieces 

 carry two, three, or four object-glasses at one time. [2, 3, 

 17, 20, 21, 22, 26.] 



Notcutt Flank Observation Computer. See Flank Observation 

 Computer. [15.] 



Notcutt Wind-Corrector. See Wind-Corrector. [15.] 



Nut Angles. Two special drawing slopes, used by mechanical 

 draughtsmen for giving the angle of hexagon nuts. [6, 15, 

 21.] 



Object-Glass. The lens or system of lenses placed at the end of 

 a telescope, microscope, etc., which is nearest to the object 

 being examined. [1, 2, 3, 8, 9, 10, 11, 12, 13, 15, 17, 18, 



20, 21, 22, 23, 24, 25, 26, 27.] 



Object-Finder. A means of registering the position of a micro- 

 scope object in a slide so that it may be readily found in 

 future. [2, 3, 17, 20, 26.] 



Object-Staff. A staff the same height as the level, forming a 

 sight to be viewed by its means, used in determining levels. 

 A levelling staff. [13, 15, 17, 24, 26, 27.] 



Objective. In optics the object-glass (q.v.). [1, 3, 10, 12, 15 



21, 26.] 



Obtuse Angle Thermometer. See Thermometer. [5, 13.] 



