186 HISTORY OF PHOTOGRAPHY. 



it8 trouble and very little of its uncertainty, and if the more frequent use of 

 it by me, as compared with other pi-ocesses, does not make me exaggerate its 

 facility of operation I think it is likely to be jiracticed successfully by the 

 most ordinary experimenters. 



Doctor Woods subsequently made the following addition: 



Since the preceding paper was written I have been experimenting with the 

 catalysotype, and one day, having had many failures, which was before quite 

 unusual with me. I am induced to mention the cause of them for the benefit 

 of subsequent experimenters. The paper I used was very stiff and highly 

 glazed, so that the solution first applied was not easil,Y imbibed. The blot- 

 ting paper was very dry and bibulous. When using the latter I removed 

 nearly all the solution of iron from the first, and of course did not obtain the 

 desired result. 



While varying the process in endeavoring to find out the cause just men- 

 tioned, I discovered that the following proportions gave very fine negative pic- 

 tures, from which good positive ones were obtained : Take of sirup of iodide 

 of iron, distilled water, each 2 drams ; tincture of iodine, 10 to 12 drops ; mix. 

 First brush this over the paper, and after the few minutes, having dried it 

 with the blotting paper, wash it over in the dark (before exposure in the camera) 

 with the following solution by means of a camel-hair pencil : Take of nitrate 

 of silver 1 dram ; pure water, 1 ounce : mix. This gives a darker picture 

 than the original preparation, and consequently one better adapted for obtain- 

 ing positive ones ; it also requires no previous steeping in an acid solution. 

 To fix the picture, let it be washed first in water, then allowed to I'emain for 

 a few minutes in a solution of iodide of potassium (5 grains to the ounce of 

 water) and Avashed in water again. The paper I use is the common unglazed 

 copy paper, but such as has a good body. I have tried the same paper with the 

 original preparation and find it to answer exceedingly well. It does not require 

 in this case, either, an acid solvition. The same precautions and hints apply 

 to the amended as to the original process, such as when it blackens in the 

 dark there is too much caustic used ; when it remains yellow or that it is stud- 

 ded with yellow spots, too much iodine ; when marked with black spots, too 

 much iron. It is necessary to mention these on account of the varying strength 

 of the materials employed. 



rERROCYANIDE OF POTASSIUM. 



At the meeting of the British Association at Plymouth in 18-11 I 

 first directed attention to the use of the ferrocyanide of potassium 

 in combination with the iodide of silver. The process resulting from 

 this being very important in mauy points, the abstract of the paper 

 then read, as given in the Transactions of the Sections, is reprinted. 



The author having been engaged in experiments on those varieties 

 of photographic drawings which are formed by the action of the 

 liydriodic salts on the darkened chloride of silver, and with a view 

 to the removal of the iodide formed by the process from the paper, 

 was led to observe some peculiar changes produced by the combined 

 influences of sunshine and the ferrocyanide of potassium. It was 

 found that the ordinary photographic paper, if allowed to darken 

 in sunshine, and then slightly acted on by any liydriodic solution, 



