CH. XV] PROJECTION OF SPECTRA 627 



Glass prism, hollow prism to hold carbon bisulphide, direct- 

 vision prism. 



Diffraction grating, ruled on speculum metal, or glass, or one of 

 the replica gratings, 6,000 to 8,000 lines to the centimeter, 

 (15,000 to 20,000 lines to the inch). 



Glass cell to hold colored liquids. 



Colored liquids, solutions of didymium nitrate, copper sulphate, 

 analine dyes, etc. 



Colored glass, especially red and blue. 



Arc lamp with vertical carbons. 



Hollow carbons stuffed with salts as: lithium chloride, sodium 

 chloride, potassium chloride, calcium chloride, aluminum. 



Flame arc electrodes, "Yellow" and "Brilliant white." 



Metallic electrodes, iron, copper, aluminum, uranium oxide in 

 tin tube, rutile in tin tube or else luminous arc electrodes 885a. 



Screen coated with anthracene, 50 x 150 cm. to use instead of 

 a white screen to show ultra-violet. 



APPARATUS FOR THE DEMONSTRATION OF ULTRA- 

 VIOLET LIGHT. 



Quartz condensing lens and two plano-convex quartz lenses. 

 Source of ultra-violet light as : 



Quartz mercury arc. 



Arc lamp with carbons filled with various salts. 



Arc lamp with carbons filled with metallic aluminum. 



Arc lamp with brilliant white flame arc carbons. 



Arc lamp with iron tube filled with uranium oxide. 

 Small card coated with anthracene to render the ultra-violet 

 visible. 



Especially since the days of Newton, the exhibition of the 

 spectrum has been one of the most fascinating experiments in 

 physics. It is also one of the simplest of the "special projection" 

 experiments to perform. 



88sa. These tubes can be made by rolling strips of tinned iron into 

 tubes. 



