92 



with much benefit, and now always use it, in quantities pro- 

 portioned to the age of the syrup. 



The following hints will, I think, enable any experimen- 

 tor to be successful in producing good pictures by this pro- 

 cess. In the first place, the paper used should be that called 

 "wove post," or well-glazed letter-paper. When the solu- 

 tions are applied to it, it should not immediately imbibe them 

 thoroughly, as would happen with the thinner sorts of paper. 

 If the acid solution is too strong, it produces the very effect 

 it was originally intended to overcome ; that is, it produces 

 yellow patches, and the picture itself is a light brick-colour, 

 on a yellow ground. When the tincture of iodine is in excess, 

 partly the same results occur ; so that if this eflfect is visible, 

 it shews that the oxide of silver which is thrown down is 

 partly re-dissolved by the excess of acid and iodine, and their 

 quantities should be diminished. On the contrary, if the sil- 

 ver solution is too strong, the oxide is deposited in the dark, 

 or by an exceedingly weak light, and in this case blackens 

 the yellow parts of the picture, which destroys it. When this 

 effect of blacking all over takes place, the silver solution should 

 be weakened. If it be too weak, the paper remains yellow 

 after exposure to light. If the ioduret of iron be used in too 

 great quantity, the picture is dotted over with black spots, 

 which afterwards change to white. If an excess of nitrate of 

 silver be used, and a photograph immediately taken before 

 the deposition of the oxyde takes place, there will be often, 

 after some time, a positive picture formed on the back of the 

 negative one. The excess of the nitrate of silver makes the 

 paper blacker where the light did not act on it, and this pene- 

 trates the paper, whereas the darkening produced by the light 

 is confined to the surface. The maximum intensity of the 

 spectrum on the paper, when a prism of crown glass is used, 

 lies between the indigo and blue ray. The difference of 

 effect of a strong and weak light is beautifully shewn in the 

 action of the spectrum : that part of the paper which is ex- 

 nosed to the indigo ray is coloured a reddish brown, and this 



