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 effect 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- 
posed to the indigo ray is coloured a reddish brown, and this 
