342 THE BEGINNINGS OF PHOTOGRAPHY. 



suggested that it was due to heat, but experiment showed it was not 

 so. He then divided the mixture into two parts, one being kept in 

 the dark while the bottle containing the other was put in the hot sun 

 with a thread passed round it al)out the middle of the part exposed to 

 the sun. After some hours' exposure the thread was removed, and 

 he was delighted to find that under it the color of the mixture was the 

 same as that in the back part of the bottle, which had not been exposed. 

 The experiment was repeated in various waj^s, and proved that the 

 change of color depended entirely on the sun's light, and that heat had 

 nothing to do with it. He then tried experiments in the converse 

 way, i. e., he mixed up the fluid to give it a uniform color, and then 

 covered the greater part of the glass with opaque ])odies, or with cut- 

 out words or sentences on paper, leaving only a small portion of the 

 mixture exposed. In this way the words or sentences were accurately 

 and distinctly reproduced on the chalk sediment, and the result was 

 looked upon as a great marvel by ignorant people. 



Feeling tliat still further invi^stigation was necessary, and l)elieving 

 that the etiects were dependent on the mixture of chalk and aqua fortis, 

 he tried several experiments with fuming spirits of niter and ordinary 

 a([ua fortis mixed with chalk, but obtaining no result, he remembered 

 that the aqua fortis he had first used contained some silver, and that 

 the effects must have been due to it, because he had already noticed 

 that solutions of silver in aqua fortis turned dark red after exposure 

 to the sun. He then repeated his first experiments with an aqua fortis 

 containing more silver, and observed that the color was more distinctly 

 marked tlian before. He found also that reflected sunlight was capable 

 of producing the same result. He notes that other white sLil)stances, 

 such as hartshorn, white magnesia, ceruse of lead, can be used to show 

 the same effect as with the chalk. Ex^n then he seems to have felt 

 that he had not penetrated to the real cause of the phenomenon, and 

 only suggests the use of it as a means of testing the presence of silver 

 in a solution. He evidentl}^ had no idea of its photographic possi- 

 bilities. 



Although Schulze did not set out witli the idea of making photo- 

 graphic copies by means of his silver and chalk mixture, and his 

 cut-out stencils were only used to give a clear demonstration of the 

 action of light, it nuist ])e acknowledged that his experiments Avore 

 distinctly photographic in that he first produces his negative images 

 of the thread, leaving a white line on a dark ground, and then the 

 positive images, dark on a wliite ground, of his cut-out words and 

 sentences, or in modern parlance his negative, or cliche. There is no 

 doubt that here we have the germ of the photographic idea, and 

 further on I shall endeavor to show how it was taken up in this coun- 

 try and led more or less directly to Wedgwood's own experiments. 



