268 ILLINOIS STATE ACADEMY OF SCIENCE 



example, the flame of carbon disulfide has a temperature 

 of only 150° C. but it is luminiscent and can affect a pho- 

 tographic plate. Pure temperature radiation at 150° 

 would be far in the infra-red and entirely devoid of any 

 photochemical action. 



Artificial illumination at present depends entirely 

 upon the emission of light from incandescent solids or 

 gases. Because of the high temperature required to 

 maintain this condition, by far the greater proportion of 

 the energy supplied is converted into heat and wasted 

 rather than emitted as light. Heat, light and electricity 

 are all forms of radiant energy; the essential distinction 

 is a difference in wave length. The visible spectrum, 

 comprised between the extreme violet and the extreme 

 red, is but a small portion of the complete spectrum. 

 Chart I shows the relation of the various forms of energy 

 radiation and the distribution of the energy from several 

 sources of radiation. 



CHART I 



Note that all wave lengths in the visible spectrum are 

 not equally visible. There is a maximum of visibility 

 nearly corresponding with the maximum of the sun's 

 radiation. The visibility curve is indicated in the small 

 rectangle. 



The sun emits a practically continuous spectrum cor- 

 responding to that from a black body at a temperature 

 of 6,000° K- which is therefore taken as the temperature 

 of the sun. Its maximum-power radiation is at a wave 

 length of 5,600 A* which is in the yellow-green of the 

 visible spectrum. About 25 per cent of the radiation of 

 the sun is in the ultra-violet, although only about 3 per 

 cent of it reaches us, the very short wave lengths being 

 absorbed or dispersed by the atmosphere. Practically 

 all the rays of wave length shorter than 3,000 A are 

 eliminated in this way. 



The radiant energy of the fire-fly is entirely within the 

 visible spectrum. This is an example of an ideal cold 

 light. It has been impossible to detect the evolution of 

 any heat from this source. 



• A=Angstrom unit=10 cm. 



