446 Mr. G. Shaw on some Photographic PhcBnomena. 



plate to the vapour of either iodine or bromine. Although 

 this fact has long been known, the nature of the action by 

 which so extraordinary an effect is produced, has not yet been 

 satisfactorily explained. In the hope of elucidating this sub- 

 ject, a series of experiments was instituted, the results of which 

 are recorded in the following remarks. 



In order to prevent misapprehension, as well as to avoid 

 circumlocution, it will be well, before proceeding to the ex- 

 periments, to describe very briefly the various stages of the 

 process by which the sensitive surfaces employed, were pre- 

 pared. A smooth surface of pure silver plated upon copper 

 is rendered perfectly clean, and is at the same time highly 

 polished by being rubbed or scoured with a mixture of pow- 

 dered tripoli and dilute nitric acid; the mixture being applied 

 to and rubbed about the plate by a pledget of cotton wool. 

 When the plate has been sufficiently treated in this way (the 

 time required for which can only be determined by experi- 

 ence), the mixture is removed by pledgets of dry wool. The 

 polish of the surface is still further improved by buffing the 

 plate with a buff of clean cotton velvet on which fine char- 

 coal powder has been sprinkled. The surface of a plate of 

 silver thus treated is highly polished, and is assumed to be 

 perfectly free from foreign matter. In order to render such 

 a plate sensitive to light, it is supported over a vessel contain- 

 ing iodine, the silver surface being of course downwards. 

 The vapour of iodine immediately attacks the surface of the 

 silver, and forms, by combining therewith, a stratum of iodide 

 of silver. By removing the plate from time to time and ex- 

 amining it by a feeble light, the relative thickness of the film 

 of iodide of silver may be ascertained by its colour. The 

 plate at first has a yellow colour, which by further exposure 

 passes to a deep gold, to an orange, and next to a purple or 

 rose colour; by still further exposure it becomes blue, and 

 passes through a series of other colours ; but as the purple or 

 rose colour indicates that thickness of the film of iodide of 

 silver most favourable to photographic experiments, it will be 

 unnecessary to follow the plate through the changes conse- 

 quent upon further exposure. This preparation of the silver 

 plate is performed in a room from which daylight is excluded, 

 or into which it is only partially admitted, for the purpose of 

 examining the colour of the plate. 



The film of iodide of silver thus procured is very sensitive 

 to light, but may be made more so by exposure to the vapour 

 of either chlorine or bromine. The quantity of either of these 

 vapours necessary to communicate the maximum sensitive- 

 ness to the iodized plate is exceedingly small ; and the pre- 



