538 SCIENCE PROGRESS 



direction in part suggested by the researches of Lummer and 

 Pringsheim. Their investigations showed that the nature of 

 the radiation emitted by a " black body " can be exactly pre- 

 dicted if the temperature of incandescence be known. It has 

 been laid down by Kirchoff that a body is capable of emitting 

 only the radiations which it absorbs : consequently a truly 

 black body absorbs and therefore emits all varieties of radiation; 

 when heated it produces a confused series of vibrations of 

 differing' frequency but, as the temperature of incandescence 

 rises, the dominant frequency increases according to a well- 

 defined law. When the temperature is low no vibration rapid 

 enough to be visible is produced ; as the temperature is raised, 

 however, a red glow appears which changes to yellow and 

 ultimately to white, as the colours at the blue end of the 

 spectrum are accentuated. 



Broadly speaking, the radiation from most incandescent 

 solid illuminants, such as the electric glow-lamp and the flat 

 flame gas-burner, resembles that of a black body. The chief 

 defect of incandescent solids is that a vast amount of non- 

 luminous energy is produced and the luminous efficiency is 

 correspondingly low; their chief advantage is that, owing to 

 the sequence of vibrations produced, we obtain a continuous 

 spectrum : consequently the colours of objects seen by their 

 light are, generally speaking, not unlike those seen by daylight. 



Lummer and Pringsheim have shown that the maximum 

 of the curve of the energy radiated by an incandescent black 

 body, at low temperatures far out into the infra-red, is advanced 

 towards the visible spectrum as the temperature increases. 

 The percentage of energy within the luminous region is there- 

 fore remarkably increased and the visible light emitted has 

 been found to be approximately proportional to the twelfth 

 power of the temperature. It is calculated that "at the enormous 

 temperature of the sun the maximum of the energy curve 

 would be located immediately over the mid-point in the visible 

 spectrum— the ideal position from the standpoint of light 

 production ; Dr. Drysdale has calculated that under solar 

 conditions as much as 50 per cent, of the energy of radiation 

 would appear in the form of visible light. 



The researches of Lummer and Pringsheim serve to show 

 the importance of the tendency of modern improvements in 

 illuminants : there has been a progressive increase in tempera- 



