137 



Phonic Wheel. See Rayleigh Synchronous Motor. \19.~] 



Phonograph. An instrument for recording and reproducing 

 sounds ; invented by Mr. T. A. Edison. The record was 

 made on a waxed cylinder by means of a style attached to a 

 membrane acted upon by the sound waves. 



Phonometer. An instrument for ascertaining the number of 

 vibrations of a given sound in a given space of time. 



Phonomotor. An instrument to illustrate the motive power of 

 sound. 



Phonorganon. An instrument designed to imitate vocal sounds 

 of speech ; a speaking machine. 



Phonoscope. An instrument for observing the motions or pro- 

 perties of sounding bodies. An apparatus for testing the 

 quality of musical strings. 



Phonoshote. An apparatus for transformation of sound into 

 light. 



Phosphoroscope. An instrument for measuring the duration of 

 phosphorescence in different substances. 



Photodrome. An apparatus consisting of a large wheel with 

 spokes which, when turning very rapidly, is illuminated by 

 momentary flashes of light passing through slits in a rotating 

 disc. 



Photographic Lens. See Lens. [1, 2, 3, 8, 9, 10, 17, 18, 20, 22, 

 23, 26.] 



Photographometer. An instrument for determining the sens- 

 ibility to luminous rays of the plates employed in photo- 

 graphic processes. 



Photo Heliograph. A modified form of telescope (q.v.) adapted 

 to taking photographs of the sun. See also Heliograph. 

 [2, 8, 13, 15, 18, 20, 21, 24, 25, 26, 27.] 



Photometer. An instrument for measuring or comparing the 

 intensity of light. There are several forms. [2, 3, 10, 13.] 



Photometer Bench. A simple optical bench (q.v.) on which the 

 photometer, and other accessories, are mounted. [3, 13.] 



Photometer, Box, Lummer Brodhun. The rays of light, from 

 the two sources, falling on two dials, non-transparent, are 

 reflected on to a prism in such a way that the image of one 

 dial appears enclosed in the image of the other dial, and 

 thus the comparison of the intensities of the light is rendered 

 easy. [2, 3.] 



Photometer, Bunsen's. The two sources of light for comparison 

 are mounted on a simple optical bench (q.v .) on each side 

 of an opaque paper screen with a greased area the grease 

 spot in the middle. [2, 10.] 



Photometer, Bunsen's Mirror. A modification of the grease spot 

 photometer in which the grease spot is viewed from both 

 sides by means of an angle mirror. 



Photometer, Dibdin's Hand. A simple and portable photometer 

 for testing street, railway, and school lights. 



