Diamond-Gauge. A gauge employed by jewellers in estimating 

 the sizes of small diamonds. 



Diaphanometer. (i) An instrument for measuring the trans- 

 parency of the air. (2) For determining the degree of 

 turbidity and depth of colour of solutions. 



Diaphanoscope. An instrument for the investigation of grass 

 seeds, barley and wheat for farinaceousness and hardness. 



Diaphanoscope. A dark box for exhibiting transparent pictures, 

 with or without a lens. 



Diaphonometer. An instrument used in testing spirits by 

 determining their transparency. 



Diaphote. An instrument designed for transmitting pictures by 

 telegraph. 



Diaphragm. (i) A plate with an opening, placed in an instrument 

 to exclude light or heat. Lines or webs may be placed across 

 the opening for the determination of definite positions. 

 (2) A thin septum for separating two spaces or bodies. 

 See also Iris-Diaphragm. [3, 8, 17, 20, 26.] 



Diaphragm-Plate. See Diaphragm. [20.] 



Diathennometer. An instrument for examining the thermal 

 resistance or the heat-conducting power of objects. 



Diatom-Prism. See Prism. [2, 3.] 



Dibdin's Hand Photometer. See Photometer. 



Dichroioscope. An instrument to exhibit the two comple- 

 mentary colours of polarised light. The quality called the 

 dichroism of crystals consists in transmitting different colours 

 when viewed in different directions. Used for testing 

 precious stones. [2, 3, 14, 17, 20, 21, 22, 26.] 



Dietherescope. An instrument for geodesy and the teaching of 

 '& optics. 



Differential Draught-Gauge. See Draught-Gauge. [6, 16, 17.] 



Differential Manometer. See Manometer. [6, 17.] 



Differential Thermometer. See Thermometer. [13, 15, 16, 17, 

 20, 21, 25.] 



Diffraction Grating. A series of fine parallel lines ruled with 

 great regularity on glass or metal for producing a spectrum 

 by the interference of light. See also Lummer-Gehrcke 

 Parallel Plate. [2, 3, 10, 14, 20, 26.] 



Diffraction Grating, Thorp. A cast in celluloid of a ruled 

 diffraction grating (q.v.). [3, 17.] 



Diffusionometer. An instrument to ascertain the rate at which 

 diffusion of gases takes place. 



Diffusion Tube. An instrument for determining the rate of 

 diffusion of different gases. It consists of a graduated tube 

 closed at one end by plaster of Paris, a substance which, 

 when moderately dry, possesses the required porosity. 



Dilatometer. An instrument for measuring the dilation or 

 expansion of a substance, especially of a fluid. [25.] 



Dines 's Anemometer. See Anemometer. [15, 17, 21, 25.] 



Dines's Meteorograph. See Meteorograph. [6, 13, 15, 17, 20, 21] 



