length of lenses suited for remedying imperfect vision, etc. 

 [2, 5, 13.] 



Opsonic Apparatus. Used for taking the opsonic index, or power 

 of resistance to disease organisms. [26.} 



Opthalmometer. (i) A surgical instrument of the nature of 

 compasses for measuring the capacity of the chambers of 

 the eye in anatomical experiments. (2) An optical instru- 

 ment for ascertaining the true distance at which an object 

 shall be viewed for the accommodation of each eye. [2, 3, 



17, 20, 26.] 



Optical Bench. A rigid, graduated bench, usually of steel, on 

 which the holders for lenses, prisms, sources of light, etc., 

 can be suitably mounted in alignment, with freedom of 

 movement along the bench, for making photometric or 

 other optical measurements. [2, 3, 4, 10, 13, 17, 19, 21, 26.] 



Optical Circle. A graduated circle used for the measuring of 

 angles in optical experiments. [1, 13, 17] 



Optical Glass. [7] 



Optical Mirror, Plane Circular. An optically worked plane 

 silvered mirror (q.v.). [1, 3, 8, 10, 11, 13, 15, 17, 18, 19, 21.] 



Optical Mirror, Plane Oval. A plane mirror of oval outline 

 placed within a Newtonian telescope for directing the light 

 from the spherical mirror into the eyepiece in a direction 

 at right angles to the line of incidence. [1, 3, 11, 13, 15, 17, 



18, 26] 



Optical Prism. Accurately worked prisms for use in optical 

 instruments. See Prism. [3, 8, 9, 10, 11, 12, 13, 15, 17, 18, 



19, 20, 26, 27.] 



Optical Projection Screen. A screen for use with a magic lantern 



having a metallic surface giving a diffuse reflection. 

 Optical Pyrometer. See Pyrometer. [4, 21, 26] 

 Optical Square. A reflecting instrument used by surveyors and 



others for laying off lines at right angles to each other. [2, 



11, 13, 15, 17, 21, 24, 25, 27.] 

 Optigraph. A form of camera used for the purpose of copying 



landscapes. 

 Optometer. An instrument for ascertaining the extent of vision 



in different individuals for the purpose of choosing proper 



lenses to correct defects of sight. [2, 17, 26.] 

 Ordnance Survey Scales. Drawing scales showing the various 



scales used in the Ordnance Survey. [2, 5, 6, 13, 15, 17, 20, 



21, 24, 25, 26, 27.] 

 Orientator. An instrument used in regulating the ground plan 



of a church so as to present the chancel to the east. [13, 15.] 

 Orograph. An instrument for mapping out undulating or 



mountainous surfaces. 

 Orrery. An apparatus which illustrates by the revolution of 



balls moved by wheelwork the relative size, periodic motions, 



etc., of bodies in the solar system. [26] 

 Orsat's Gas Analysis Apparatus. See Gas Analysis Apparatus. 



I 2 



