A Manual Of Photography

TUE CATALYSOTYPE. 81 «- My reason for using the muriatic solution previous to washing with the loduret of iron is this : I was for a long time tormented by seeing the pictures spoiled by yellow patches, and could not remedy it, until I observed that they presented an appear- ance as if that portion of the nitrate of silver which was not de- comvosed by the loduret of iron had flowed away from the part. I™hen recollected that Sir J; Herschel and Mr. Hunt had proved that iodide of silver is not very sensitive to light, unless some free nitrate be present. , I accordingly tried to keep both together on the paper, and after many plans had failed, 1 suc- ceeded by steeping it in the acid solution, which makes it freely and evenly imbibe whatever fluid is presented to it. 1 am sure that its utility is not confined to this efiect, but it was for that purpose that I first employed it. “My reason for adding the tincture of iodine to the syrup is, that having in my first experiments made use of, with success, a syrup that had been for some time prepared, and afterwards remarking that fresh syrup did not answer so well, I exammed both, and found in the former a little free iodine,I therefor added a little tincture of iodine with much benefit, and now always use it in quantities proportioned to the age of the syrup. “ The following hints will, I think, enable any experimenter to be successful in producing good pictures by this process. In e first place, the paper used should be that called wove post, or well-glazed letter paper. When the solutions 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, • o that if this effect is visible, it shows that the oxide of silver which is thrown down is partly re-dissolved by the excess of acu and iodine, and their quantities should be diminished. On the contrary, if the silver solution is too strong, the oxide is depo- sited in the dark, or by an exceedingly weak light, and in ithis case blackens the yellow parts of the picture, which destroys it. When this effect of blackening all over takes place, the silver so- lution should be weakened. If it be too weak, the paper remains vellow after exposure to light. If the loduret 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 immediate y taken before the deposition of the oxide takes place, there will be often after some time a positive picture formed on the back of the negative one. G