THE PRODUCTION OP POSITIVES. 159 



formation of the ferric salt from use and exposure 

 tends to slow the process with consequent weakening 

 of the image, to say nothing whatever of its de- 

 structive effect on the silver compounds which form 

 the image. 



These positives may be intensified by mercury and 

 ammonium hydrate in the same way as ordinary 

 dry plates, but we do not recommend this method, 

 as the prints are liable to turn yellow on the applica- 

 tion of a hot iron. 



Enlarging. There are innumerable methods em- 

 ployed for this purpose, we select two as being most 

 suitable for our work, and requiring very little ad- 

 ditional apparatus. 



The first method is identical with that last de- 

 scribed, except that the lenses composing the ob- 

 jective are reversed, i.e., the front lenses are placed in 

 the cell containing the back combination, with the 

 convex surface towards the screen, while the back 

 combination is placed in the cell, from which the 

 front lenses have been removed, so that the lens which 

 formerly faced the screen, now faces the object to be 

 enlarged. If the cells themselves will fit either 

 end of the tube, this reversal is easily accomplished 

 without removing the lenses. It should always be 

 remembered, that the lenses forming what is con- 

 sidered as the back combination in ordinary photo- 

 graphy, must be placed in close apposition, and the 

 ring which separates them placed in the cell before 

 them. 



