LUMINESCENCE AND INCANDESCENCE 21 



them. Thus, a white-hot object liberates radiant energy 

 or flux of many different wave-lengths corresponding to 

 what we know as ' ' heat, light and actinic rays. ' ' All can 

 be dispersed by prisms of one or another appropriate 

 material to form a wide continuous spectrum, such as 

 that indicated in Fig. 1. Eadiant energy of A = .76/* to 

 A = .4/*, evaluated according to its capacity to produce 

 the sensation of light, is spoken of as visible radiation 

 or luminous flux. 



Below the infra-red comes a region of wave-length as 

 yet uninvestigated, and beyond this may be placed the 

 Hertzian electric waves of long wave-length used in wire- 

 less telegraphy. Above the ultra-violet comes another 

 region as yet uninvestigated, and then Rontgen rays 

 (X-rays) and radium rays, of exceedingly short wave- 

 length. These last types need not concern us except in 

 that we may later inquire if they are given off by luminous 

 animals. The shortest of the ultra-violet are known as 

 Schumann and Lyman rays. These relations are brought 

 out in Table 2. 



TABLE 2. 

 Wave-lengths of Various Kinds of Radiation 

 Wave-lengtha of light are visually given in Angstrom units. One 

 micron (;«)=. 001 mm. = 1000 millimicrons (/*,(*)= 10,000 Angstrom units 

 (A) or tenth metres = 10-"° metres or 10-' centimetres. The entire scale 

 of wave-lengths extends from 10° to 10-° centimetres. 



Hertzian electric vcaves (upper limit not reached) ahove 

 12 km. to 16 cm. 



Unexplored region 16 cm. to SlO/i 



Infra-red 310;" to .76/* 



Visible light 7600 A to 4000 A 



Ultra-violet 4000 A to 320 A 



Unexplored region 320A to 12 A 



X-rays 12 A to 0.2 A 



Eadium 7 rays 0.2 A and shorter 



