CH. xxxiv. SPECTRUM ANALYSIS. 33* 



this, took a thermometer and passed it gradually from one 

 end to the other of the coloured band. The result was 

 curious. He began at the violet end of the spectrum (Plate 

 I. No. i, p. 330), and, as he expected, the thermometer rose' 

 higher and higher as he approached the yellow part; but 

 to his surprise it did not stop here. When he passed on 

 through the yellow into the red, the heat still increased, 

 and even became more intense as he passed out of the 

 coloured band altogether into the darkness beyond. By 

 this experiment he found that the heat-rays extend for some 

 distance beyond the red colcur, and that they are strongest 

 in that part where no light is to be seen. 



Discovery of Chemical Bays by Hitter, 1801. 

 Soon after Sir William Herschel had discovered the dark 

 heat-rays, a still more remarkable fact was brought to light 

 about the violet end of the spectrum. The Danish chemist 

 Scheele, whom you will remember as one of the discoverers 

 of oxygen (see p. 230), had once remarked that nitrate of 

 silver will turn black if the violet rays of a spectrum are 

 thrown upon it. In 1801 Professor Ritter, of Jena, was 

 repeating this experiment, and he found that the black 

 patches appeared slightly on those parts of the paper where 

 the violet rays fell, but very strongly indeed beyond those 

 rays where the spectrum was quite dark. So that at this 

 end also there are invisible rays, and these have the extra- 

 ordinary power of decomposing or breaking up nitrate of 

 silver, and some other substances, so as to leave distinct 

 marks upon anything touched by them. 



Photography. You will see at once that this is the 

 secret of Photography. In 1802, Sir Humphry Davy and 

 Dr. Thomas Wedgwood suggested that pictures might be 

 taken in this way by the rays of the sun acting upon chloride 

 of silver, and they even succeeded in making some. But 



