RADIATION. 181 



solution of iodine only the dark rays are transmitted. 

 Determining, then, by means of a thermo-electric pile, the 

 total radiation, and deducting from it the purely obscure, 

 we obtain the amount of the purely luminous emission. 

 Experiments, performed in this way, prove that if all the 

 visible rays of the electric light were converged to a focus 

 of dazzling brilliancy, its heat would only be one-ninth of 

 that produced at the unseen focus of the invisible rays. 



Exposing his thermometers to the successive colors of 

 the solar spectrum, Sir William Herschel determined the 

 heating power of each, and also that of the region beyond 

 the extreme red. Then drawing a straight line to represent 

 the length of the spectrum, he erected, at various points, 

 perpendiculars to represent the calorific intensity existing 

 at those points. Uniting the ends of all his perpendiculars, 

 he obtained a curve which showed at a glance the manner 

 in which the heat was distributed in the solar spectrum. 

 Professor Mliller, of Freiburg, with improved instruments, 

 afterward made similar experiments, and constructed a 

 more accurate diagram of the same kind. We have now to 

 examine the distribution of heat in the spectrum of the 

 electric light ; and for this purpose we shall employ a par- 

 ticular form of the thermo-electric pile, devised by Melloni. 

 Its face is a rectangle, which by means of movable side- 

 pieces can be rendered as narrow as desired. We can, for 

 example, have the face of the pile the tenth, the hundredth, 

 or even the thousandth of an inch in breadth. By means 

 of an endless screw, this 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 gradually moves from the violet 



