Polariscope, Norrenberg's. A polariscope for exhibiting the 

 interference figures of crystals under convergent polarised 

 light. [3, 26.] 

 Polemoscope. An opera or field glass with an oblique mirror 



arranged for seeing objects that do not lie directly before 



the eye. [13.] 



Polygonoscope. A form of kaleidoscope (q.v.) [2, 20, 26.] 

 Polymeter. A sensitive and accurate form of hair hygrometer 



(q.v.). [2, 15, 16, 17, 21, 25, 26, 27.] 

 Polyoptrum. A lens, one side of which is plane and the other 



convex. 



Polyphase Slide-Rule. -See Slide-Rule. 

 Polyscope. A multiplying lens (q.v.). 

 Porro Prism Erector. A pair of right-angle prisms as used in 



prismatic binocular field-glasses to erect an image in two 



meridians. Invented by Professor Porro. See Prism, 



Erecting. [1, 2, 3, 10, 13, 15, 20, 26, 27] 

 Portrait Lens. See Lens. 

 Position-Finder. An instrument used for ascertaining the 



position of an object on a map, or the position of a ship on 



the chart. [2, 13, 15, 16, 26, 27, 28.] 

 Position Micrometer. A micrometer applied to the tube of an 



astronomical telescope for measuring angles of position in 



the field of view. [2, 3, 13, 25, 27.] 

 Positive Eyepiece. See Eyepiece. 

 Pot Leather Thermometer. See Thermometer. [6, 13.] 

 Potentiometer. An instrument for measuring the potential 



between two points in a circuit traversed by an electric 



current. [4, 16, 19.] 

 Potometer. For measuring and demonstrating the relative rates 



of transpiration of the same plant under different external 



conditions. [8.] 

 Potometer. Ganong 's . 

 Potometer. Pethybridge's. 



Praxinoscope. An instrument for presenting to view or pro- 

 jecting upon a screen images having the natural motions 



of real objects. [26] 

 Pressler Borer. An instrument for ascertaining the rate of 



growth of trees. [21.] 

 Pressure Gauge. For indicating pressure of steam, etc. See 



Gauge. [4, 5, 6, 13, 15, 16, 17, 21, 26, 27, 28.] 

 Pressure Gauge, Air. See Gauge. [4] 

 Pressure Gauge, Bourdon. See Gauge. [4.] 

 Pressure Gauge, Edney. See Gauge. [4.] 

 Pressure Head. Another name for Pitot tube (q.v.). [4, 6, 17] 

 Pressure Thermometer. See Thermometer. [4, 5, 6, 13, 15, 17.] 

 Pricker. A drawing instrument. [5, 6, 13, 15, 17, 20, 21, 24, 27.] 

 Prime Vertical Transit Instrument. See Transit. [21, 25.] 



