143 



Prism, Roof. A reflecting prism in which one of the reflecting 



surfaces is replaced by a pair at right angles in the form of 



the gable of the roof ; used to reverse the image in two 



meridians. [1, 2. 3, 8, 9, 12, 13, 14, 15, 18, 20, 23, 26.'] 

 Prism, Rutherford's. A double-image prism used in polarising 



experiments. [3, 13, 26.] 

 Prism, Thompson. A polarising prism, invented by Prof. Sil- 



vanus Thompson, passing a very wide angle of light. [2, 3, 



22, 26.] 

 Prismatic Compass. See Compass. [2, 5, 9, 10, 13, 15, 16, 17, 



18, 20, 21. 24, 25, 26, 27.] 

 Prisms for Spectroscopes. Optically worked prisms for use in 



spectroscopes. [2, 3, 8, 10, 12, 13, 14, 26] 

 Process Lens. A lens (q.v.) used in process reproductions. 

 Process Prism. A prism used in process reproduction. 

 Proell's Pocket Calculator. A simple form of slide-rule card 



with an upper sheet of transparent celluloid. [8, 13, 15] 

 Projection Apparatus. A magic or optical lantern used for 



projecting an enlarged image of an illuminated object on a 



screen of suitable surface. [2, 3, 8, 10, 20, 26] 

 Proof Glass or Plate. A glass plate with a perfectly plane or 



spherical surface for testing the accuracy of surfaces by 



means of the interference of light ; used especially in glass- 

 polishing. [1, 3, 8, 10, 11, 20, 22, 23, 26.] 

 Propeller-speed Indicator. An instrument for indicating the 



revolutions of a propeller. See also Speedometer. [15, 17] 

 Proportional Compasses. See Compasses. [2, 5, 6, 13, 15, 16, 



17, 20, 21, 24, 25, 26, 27.] 

 Protractor. A mathematical instrument for laying down and 



measuring angles on paper. [2, 5, 6, 13, 15, 16, 17, 20, 21, 



24, 25, 26, 27, 28] 

 Protractor. Set-square pattern. [5, 6, 13, 15, 16, 17, 20, 21, 



24, 25, 26, 27] 

 Protractor. Vernier pattern. A protractor with a radial arm 



and vernier. [5, 6, 13, 15, 16, 20, 21, 24, 25, 26, 27] 

 Protractor, Chart. A circular, semicircular, or rectangular scale 



divided in degrees for laying off the ship's course on a 



chart. [13, 15, 16, 21, 28] 

 Protractor, Creagh-Osborne. A specially designed protractor for 



use with the Creagh-Osborne compass (q.v.). [13, 15, 20, 



26, 27] 

 Protractor, Isometrical. A protractor constructed for isometrical 



perspective drawing to give all angles from the horizontal 



in correct ratio. [27.] 

 Protractor, Sutherland's. A special form of protractor designed 



by Mr. Angus Sutherland. It is used in conjunction with 



angular instruments in order that the survey may be drawn 



on the tracing paper attached to it at the actual place of 



observation. [13] 



