1873. J Actinism and Magnetism. 301 



be the disengagement of the iodine from the ultimates of 

 silver near the surface, where the vibratory action is 

 greatest — so allowing it to penetrate inwards and attach 

 itself to those silver ultimates, which are less agitated by 

 the motion. The iodine thus, as it were, eats its way in- 

 ward, leaving behind it ultimates of silver more or less dis- 

 engaged. Accordingly, when the plate is exposed to the 

 action of mercurial vapour, the ultimates of mercury attach 

 themselves to those disengaged ultimates of silver, forming 

 with them a white amalgam, which constitutes the lights of 

 the picture — the unamalgamated parts of the silver forming 

 the shadows ; while the unaltered film of iodide of silver is 

 removed by the hyposulphite of soda. 



The latent image in the collodion processes presents two 

 cases — the one that of the wet film, the other that of the 

 dry. The case of the wet film resembles that of the 

 Daguerreotype. The silver ultimate and the iodine ulti- 

 mate, which it retains near it by its attraction, begin to 

 vibrate under the stimulus of the incident light. The 

 parargyrine and pariodine in the intervening space are thus 

 forced to intermingle — so weakening the attraction between 

 the two ultimates. This condition continues for a con- 

 siderable time after the stimulus of the external light is 

 withdrawn. When a developer is applied, it takes advan- 

 tage of the moment of greatest weakness, when the ulti- 

 mates of silver and iodine are in the course of their 

 vibration farthest asunder, and it effects their separation, 

 the iodine combining with the developer, while the silver 

 resumes the metallic form. The deposit of silver in this 

 case constitutes the shadows of the picture when it is viewed 

 as a transparency, and it is then accordingly negative ; but 

 when the picture is viewed by reflected light with a piece of 

 black velvet behind it, the deposit of silver forms the lights 

 of the picture which is then positive, and such are some of 

 the most pleasing photographs. 



When the collodion film is dry, again, it requires more 

 applied energy to establish the vibratory condition as be- 

 tween the silver and the iodine ultimates, owing to the 

 rigidity of the film ; and for the same reason the vibrations 

 are probably arrested the moment that the stimulus of the 

 external light is withdrawn. But the attraction of the 

 silver for the iodine has been permanently weakened 

 through the action of the light, by reason of the re- 

 admixture of the parargyrine with the pariodine, in the 

 interval between them, resulting from the vibrations. 

 Hence, at the moment of the arrest of motion, the ulti- 



vol. in. (n.s.) 2 R 



