THE BEGINNINGS OF PHOTOGEAPHY. 343 



1)U FAY AND irELLOT. 



In the M(unoir,s of the French Acadeni}^ for 1728, page 50, Du Fa}^ 

 has de.scri])ed a method of stainiiio- agates ))y treating" them with a 

 sohition of silver nitrate, and when dry exposing them to sunshine; 

 the solution, penetrating to different depths in the more or less al)sorb- 

 ent layers of the stone, produced variegated effects not shown in the 

 original. In some cases the solution was applied with a pen. Du Fay 

 seems, however, to have had no idea of using a stencil, and to have 

 worked, though almost contemporaneously, cpiite independently of 

 an3'thing done l)y Schidze, the staining of objects b}^ silver nitrate 

 solutions })eing, as we have already seen, w^ell known, though the 

 necessity of the objects being exposed to light was not so clearly 

 recognized. He also used solutions of gold and bisnuith, and notes 

 the favorable effect of a certain amount of moisture in strengthening 

 the reduction. 



Similarly, llellot, in the same memoirs for 1787, mentions the use of 

 a weak solution of silver nitrate as a sympathetic ink which would 

 show nothing so long as the paper were kept in darkness, ])ut on 

 exposure to the sun darkened and showed the writing in a slaty gray, 

 this effect being due, as Hellot thought, to the action of some sulphur- 

 ous principle in the nitric acid which ])lackened the silver. This is 

 int('r(\sting as, apparentl}', the first recorded graphic application of 

 silver nitrate to paper. 



BECCARl's OBSEKVATIONS. 



We have already pointed out the incorrectness of the commonh' 

 accepted statement that Georgius Fabricius was the ffrst to pu])lish the 

 fact that luna cornea, or silver chloride, darkened on exposure to 

 light, and, although this darkening must have been constantly observed, 

 it was, even up to Boyle's time, not attributed to the action of light, 

 but rather to sofiio effect of the air or sulphurous vapors. That it was 

 due to the action of light was first proved by Jacopo Bartolomeo Bec- 

 cari, of Bologna, in 1757, b}^ a method ver}'^ similar to Schulze's with 

 the nitrate. His paper, in the fourth volume of the Commentaries of 

 the Bolognian Academv, deals with the power which light possesses of 

 itself to change not only the colors but likewise the texture of things. 



Having a suspicion that the change in color of luna cornea, gener- 

 ally attributed to the action of the air, was due to light, he inclosed 

 some in a glass vessel and i)laced it at some distance in front of the 

 window of a room not ver}^ brilliantly lighted. After some time he 

 noticed that the luna cornea on the side toward the window had turned 

 violet, while that on the other side away from the light retained its 

 original color. This showed that there was some influence in light 

 which caused changes of color. To make ((uite sure, however, ho tixed 

 SM V.HrS 23 



