44 FRAGMENTS OF SCIENCE. 



linear thermo-electric pile may be moved through the 

 entire spectrum, from the violet to the red, the amount 

 of heat falling upon the pile at every point of its march, 

 being declared by a magnetic needle associated with 

 the pile. 



When this instrument is brought up to the violet 

 end of the spectrum of the electric light, the heat is 

 found to be insensible. As the pile is gradually moved 

 from the violet end towards the red, heat soon mani- 

 fests itself, augmenting as we approach the red. Of all 

 the colours of the visible spectrum the red possesses the 

 highest heating power. On pushing the pile into the 

 dark region beyond the red, the heat, instead of vanish- 

 ing, rises suddenly and enormously in intensity, until 

 at some distance beyond the red it attains a maximum. 

 Moving the pile still forward, the thermal power falls, 

 somewhat more rapidly than it rose. It then gradually 

 shades away, but, for a distance beyond the red greater 

 than the length of the whole visible spectrum, signs of 

 heat may be detected. 



Drawing a datum line, and erecting along it per- 

 pendiculars, proportional in length to the thermal in- 

 tensity at the respective points, W T C obtain the extraor- 

 dinary curve, shown on the opposite page, which ex- 

 hibits the distribution of heat in the spectrum of the 

 electric light. In the region of dark rays, beyond the 

 red, the curve shoots up to B, in a steep and massive 

 peak a kind of Matterhorn of heat, which dwarfs the 

 portion of the diagram c D E, representing the luminous 

 radiation. Indeed the idea forced upon the mind by 

 this diagram is that the light rays are a mere insigni- 

 ficant appendage to the heat-rays represented by the 

 area A B c D, thrown in as it were by nature for the pur- 

 pose of vision. 



The diagram drawn by Professor Miiller to repre- 



