136 



Pendulum, Simple. An instrument consisting essentially of a 

 flight string or wire from which is suspended a heavy mass 



or " bob," free to swing in a vertical plane. 

 Pensky-Marten's Flash-Point Apparatus. See Rash-Point Ap- 

 paratus. [15, 17.] 

 Pentagonal Prism. See Prism. [1. 2, 3, 8, 10, 12, 13, 14, 18, 20, 



26, 27.] 

 Perambulator. A very ancient instrument described by Vitruvius 



as being among the effects of the Emperor Commodus. It 



was used, by hand or attached to a carriage, to measure 



distances. [6, 27.] 

 Perambulator Wheel. An instrument used in surveying for 



measuring distances over land or roads. [2, 5, 13, 15, 16, 



17, 21, 24, 26, 27.] 



Perfect Speed Indicator. See Speedometer. [5, 6, 13, 15, 17.] 

 Perfect Speedometer. See Speedometer. [5, 6, 13, 15, 17.] 

 Perimeter. (i) An instrument for the determination of the 



angle of strabismus. (2) An instrument for determining 



the area of sensitivity of the retina. [2, 26.] 

 Periscope. An instrument, generally in the form of a bent 



telescope, for viewing objects at a distance from a sheltered 



position, where direct vision is impossible, [1, 3, 5, 9, 11, 



13, 15, 16, 17, 20, 21, 26, 27.] 

 Periscope, Submarine Vessel type. A special type of periscope 



for use on submarine vessels. 



Periscopic Lens. (i) A meniscus lens. (2) A form of non- 

 achromatic photographic lens consisting of two meniscus 



lenses separated by a considerable interval. [3, 8, 10, 20, 



26, 27.] 



Periscopic Spectacles. See Spectacles. [20, 21, 26.] 

 Permeability Tester. An instrument for testing the permeability 



of fabrics used in airships and aeroplanes. [4.] 

 Permeameter. Same as permeability tester. [4.] 

 Perspectograph. An instrument for the mechanical drawing of 



objects in perspective. 

 Petrol Tank Gauge. For indicating the amount of petrol in 



the tank of an aeroplane, etc. [6, 15.] 



Petroleum Test Apparatus. See Flash-Point Apparatus. [15, 21.] 

 Phakoscope. See Phantascope. 

 Phanarogrisonmeter. An apparatus for indicating the presence 



of dangerous gases in mines. 

 Phantascope. An optical instrument for observing the reflected 



images seen in the human eye when it is being accommodated 



to a near object. 

 Phenakistoscope. An instrument depending, like the thauma- 



trope and zoetrope (q.v.), upon the persistence of visual 



impressions on the retina. 

 Phonautograph. See Phonograph. 

 Phoneidoscope. An instrument for observing the colour figures 



of liquid films when acted on by sonorous vibrations. [19.] 



