Integrator. An instrument giving area and moment upon any 

 diagram, but not the moment of inertia. [13, 15, 17, 21, 24, 

 27, 28.-] 



Interference Apparatus. An apparatus for studying the inter- 

 ference of vibration, generally applied to Koenig's tuning- 

 forks and revolving mirrors for studying sound. [26.} 



Interferometer, Michelson's. An instrument for making very 

 accurate measurements of small distances in terms of the 

 wave-length of light. It consists essentially of four plates 

 of glass so arranged as to be equivalent to two parallel 

 reflecting surfaces, the distance between which can be 

 varied. A beam of light is split up into two components 

 which are recombined after one of them has travelled a 

 small number of wave-lengths further than the other, thus 

 producing interference fringes. [3, 14, 25, 26.] 



Interferometer, Fabry and Perot. A form of interferometer 

 (q.v.) based on the principle of multiple reflections, by which 

 the width of the bright bands is made many times smaller 

 than the width of the dark bands. It consists essentially of 

 two plates of plane-parallel glass, one stationary, the other 

 movable, the opposed surfaces of the glass plates being 

 half -silvered. 



Interferometer, Hilger. An interferometer for testing the accur- 

 acy of optical glasswork. [14.] 



Interferometer, Lummer and Gehrcke. An interferometer, re- 

 sembling in principle the echelon spectroscope, for ob- 

 taining narrow interference fringes under conditions of large 

 retardation of path. 



Invar Measuring Tape. A measuring tape made of the metallic 

 alloy known as Invar which has no perceptible expansion 

 when heated. [6, 13, 21, 24, 27.} 



Inward's Parabolagraph. See Parabolagraph. 



Iriscope. An instrument for exhibiting the prismatic colours of 

 light. 



Iris-Diaphragm. A contractile diaphragm, simulating the action 

 of the iris of the eye, to regulate the size of the aperture 

 in an instrument through which light passes. [1, 2, 3, 8, 17, 

 20, 23, 26, 27.] 



Isogon. An instrument for making a bevel square. By it 

 an angle may be set at once by the degrees and the same 

 angle may be reversed. [15] 



Isograph. An adjustable set-square, accessory to the tee-square, 

 used principally for drawing slopes of roofs, spires, cones, 

 etc. [2, 13, 15, 21, 24, 27] 



Isometrical Protractor. See Protractor. [27] 



Jacob's Staff. A pole with iron shoe used as a stand for a 

 prismatic compass, box sextant, or clinometer. [2, 13, 15, 

 17, 21, 24, 25, 27] 



Jakin Slide-Rule. See Slide-Rule. [13, 15] 



