2 LECTUEES TO SCIENCE TEACHERS. 



We shall only cast a rapid glance over the early history of 

 photography, and endeavour to show as far as possible how 

 it was that the great advances in it have been made. 



In 1777 Scheele, of Stralsund, in Sweden, was the first 

 who actually carried out investigations into the action of light 

 on silver chloride. Before his time it was well known that 

 " luna cornua " (as silver chloride was termed) blackened in 

 the presence of light, but he arrived at the fact that a chemi- 

 cal cbange wa^s brought about by the light. He found that 

 silver chloride blackened by this agency on being treated 

 \viVn ammonia yielded up metallic silver, whilst if the ex- 

 posure cook place beneath water a soluble substance was 

 separated, which, when silver nitrate was applied, gave 

 fresh silver chloride. This was an important investigation, 

 but no fruits resulted from it till many years later. 



Wedgwood, in 1802, next called attention to photographic 

 action, in a paper read before the Eoyal Institution, entitled, 

 "An Account of a Method of Copying Paintings on Glass, 

 and of making Profiles by the Agency of Light upon Nitrate 

 of Silver," with Observations by H. Davy. This was the 

 first published account of producing photographs. 



Wedgwood used white leather or white paper as a sub- 

 stratum (using the former in preference to the latter) on 

 which was brushed silver nitrate. In his paper he entered 

 into details of his process, and admitted that the images so 

 obtained could not be fixed or rendered permanent. Davy 

 compared Wedgwood's results when using silver nitrate, with 

 those obtained by silver chloride, and found that the latter 

 compound was more susceptible to darkening by the action 

 of light than the former ; but in neither case could he fix the 

 images. 



The next eminent man who made essays on what we now 

 call photography (or sun-writing) was Nicephore de Mepce. 

 He commenced his experiments in 1814, and in 1827 

 wished to communicate an account of them to the Eoyal 

 Society of London ; his paper was not received, owing to the 

 details of the process being kept a secret. We now know 

 that his process was founded on the action that light pro- 

 duced on bitumen of Judaea (more commonly known, perhaps, 

 as asphaltum) ; he found tbat this body when exposed to light, 

 became insoluble in the usual menstrua. Thus, if a thin 

 coating were given to a metal plate and when dry exposed 



