as Photographic Agents. 265 



same solutions, the glasses being connected with a piece of 

 cotton, as in the adjoining figure. 



A few hours of daylight occa- 

 sioned the hydriodic solution in the 

 tube to become quite brown with 

 liberated iodine; a small portion 

 of the iodide of silver was formed 

 along the cotton, and at the end 

 dipping in the salt of silver. (The 

 glasses were kept purposely wide 

 apart to prevent a quick formation.) 

 During the night the hydriodic liquid became again colourless 

 and transparent, and the dark salt along the cotton was as 

 yellow as at fast. 



64-. A curious illustration of the action I have endeavoured 

 to elucidate, may be had by operating with chlorine. Its first 

 action on one of these hydriodic photographs is the separation 

 of iodine, which, when exposed to the light, is seen to act on 

 the edges first, and gradually over the whole extent of the 

 darkened portions. This curious action may be repeated 

 until all the iodine is removed from the paper. 



I think it will now be evident, that before we can expect to 

 have quite permanent and well-finished hydriodic photographs, 

 we must have at command the means of removing all the iodide 

 of silver without injuring the dark oxide. 



65. On the Action of the dissevered Rays of the Solar Spec- 

 trum on dark photographic Papers washed with an hydriodatc 

 Liquid. Sir John Herschel, in his valuable memoir before- 

 mentioned, has clearly shown, " that the total effect of a ray 

 of white light on iodic preparations, is in fact the difference 

 of two opposing actions, either of which may be exalted or 

 enfeebled at pleasure by circumstances under our command, 

 but difficult to reproduce exactly at our pleasure. When 

 these opposing actions," I still quote Sir John's words, " ex- 

 actly neutralize each other, the paper is insensible. When 

 either preponderate, it is positive or negative in its charac- 

 ter, according to that of the preponderant action ; nay, it 

 may at one and the same moment be positive to light inci- 

 dent under certain circumstances, insensible under others, and 

 negative under a yet different illumination." 



66. These singular facts were noticed by me in a very 

 early stage of my inquiries ; and you may perhaps remember 

 my forwai'ding to you, with some specimens illustrative of my 

 paper on " The Chemical Action of the Solar Spectrum," an 

 hydriodated photograph, which exhibited the effects of co- 

 loured media in determining the action*. 



* See Sir John Herschel's memoir " On the Chemical Action of the 

 Rays of the Solar Spectrum," Phil. Trans. 1840, Part i. page 43. 



