GENERAL CONSIDERATIONS. , 9 



3. Of the harmony or discord of simultaneous sounds. 



4. Of the melodious succession of sounds. 



5. Of the timbre of sounds, and of the distinction of vowel 



sounds. 

 6. Of the time required for the perception of the sensation 



of sound. 



RADIATION. 

 PHYSICAL ASPECT OF OPTICS. 



i. Sources of Radiation., Heated bodies, solid, liquid, and 



gaseous. 

 Solid. Heated by a blow-pipe as in the oxy-hydrogen 



limelight. 



Heated by their own combustion, as in the mag- 

 nesium light and glowing coals. 

 Heated by an electric current, as the carbon 



electrodes of the electric lamp. . . 

 Heated by concentrated radiation from other 



sources, as in the phenomenon called Cal- 

 escence. 



Liquid, as in hot fused metals and other bodies. 

 Gaseous. Heated by their own combustion, as in flames. 

 Heated by a Bunsen burner, as the sodium 



light. 



Heated by the voltaic arc. 

 Heated by the induction spark. 



2. Distributors. Burning mirrors and lenses, condensing 

 lenses for solar microscopes, magic lanterns, &c., 

 lighthouse apparatus ; telescopes, microscopes, &c. 

 3. Selectors. Absorbing media and coloured bodies in 

 general, prisms and spectroscopes, ruled gratings, &c. : 

 tourmalines, NicoPs prisms, and other polarizers. 

 4. Phosphorescent, fluorescent, and calescent bodies. 

 5. Opaque screens, diaphragms, and slits. 

 c 2 



