144 GREAT MEN OF SCIENCE 



silver compound by means of light; he proved that the black 

 is finely divided silver.^ 



Using pure chloride of silver, he observed the blackening 

 of it in the spectrum of the sun produced by a glass prism, 

 and found that it was stronger in the violet than in the other 

 colours. When J. W. Ritter, of Jena, repeated the experi- 

 ment in 1802, in a somewhat more refined manner, he 

 noticed that the blackening extends considerably beyond the 

 violet end of the spectrum; he thus discovered the invisible 

 ultra-violet rays. 



Joseph Priestley was born in a village near Leeds in 

 England; he became a clergyman, and was occupied in turn as 

 a preacher and teacher, but he always busied himself with 

 scientific studies apart from his occupation. His idea of 

 Christianity was essentially different from that general in his 

 country, and this produced many enemies for him.^ 



When in the year 1791, the second anniversary of the out- 

 break of the French Revolution was celebrated (the deepest 

 causes of which were certainly unknown to Priestley), he 

 gave unchecked expression to his approval, and his enemies 

 seized the opportunity to stir the mob up and destroy his 

 house and laboratory near Birmingham, where he lived at 

 that time, and to plunder it and burn it down. Priestley 

 was scarcely able to save himself and his family, and no 

 longer finding sufficient security in his own country, he 

 emigrated to America, where he died nine years later at the 

 age of seventy-one. In spite of all this, he had faithful friends 



1 The art of photography has developed on the basis of these funda- 

 mental facts by gradual addition of all sorts of devices, to a continually 

 increasing degree of refinement, without any fundamentally new facts 

 having been discovered. The first fixation of a photographic image pro- 

 duced by short exposure and subsequent development was effected by 

 the painter Daguerre in Paris, in 1835. 



2 He also published in 1782 a work called History of the Corruptions 

 of Christianity (he had also lectured upon this theme previously), in 

 which he devoted great pains to arrive at more truth concerning the life 

 and doctrine of the Founder of Christianity, than was usually accepted. 



