HISTORY OF PHOTOGRAPHY. 185 



better word, the " catalysotype." Sir J. Herschel and Mr. Fox Talbot have 

 remarked the same fact with regard to other salts of iron, but I do not know 

 of any process being employed for photographic purposes which depends on 

 this action for its development except my own. 



My reason for using the nuiriatic solution previous to washing with the iodide 

 of iron is this : I was for a long time tormented by seeing the pictures spoiled 

 by yellow patches and could not remedy it luitil 1\ observed that they presented 

 an appearance as if that i)ortion of the nitrate of silver which was not decom- 

 posed by the iodide of iron had flowed away from the part. I then 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 I succeeded 

 by steeping it in the acid solution, which makes it freely and evenly imbibe 

 whatever tluid is jiresented to it. I am sure that its utility is not confined to 

 this effect, but it was for that purpo!«ie that I first employed it. 



My reason for adding the tincture of iodine to the sirup is, that having in my 

 first experiments made use of, with success, a sirup that had been for some time 

 prepared, and afterwards remarking that fi-esh sirup did not answer so well, I 

 examined both and found in the former a little free iodine. I therefore added 

 a little tincture of iodine with much benefit, and now always use it in (luanti- 

 ties proportioned to the age of the sirup. 



The following hints will, I think, enable any experimenter to be successful 

 in producing good pictures by this process : In the first place, the paper used 

 should be that called " wove post," or well-glazed letter paper. When the solu- 

 tions are ai)plied 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 color 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 shows that the oxide of silver which 

 is thrown down is partly redissolved by the excess of acid and iodine, and their 

 quantities should be diminished. On the contrary, if the silver solution is too 

 strong, the oxide is dei)osited 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 blackening 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 iodide 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 oxide takes place, there will bo 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 penetrates 

 the paper, whereas the darkening jiroduced by the light is confined to the sur- 

 face. The maximum intensity of the spectrum on the paper when a ])risni of 

 crown glass is used lies between the indigo and blue ray. The difference of 

 effect of a strong and weak light is beautifully shown in the action of the 

 spectrum ; that part of the paper which is exposed to the indigo ray is colored 

 a reddish brown, and this is gradually darkened toward either extremity until 

 it becomes a deep black. 



I have not had many oi)portuiuties of expei-imenting with the catalysotype, 



but it certainly promises to rei)ay the trouble of further investigation. The 



s-implicity of the process and the sensibility of the pai)er should cause it to be 



extensively used. It has all the beauty and quickness of the calotype, without 



SM 1905 16 



