MEMOIK XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 



419 



From this it appears that the sensitiveness of this form 

 of iodide depends not merely on its chemical constitution, 

 but also on its optical properties. The explanation of 

 this different sensitiveness in different films of iodide be- 

 comes obvious when we cause a tablet prepared, as just 

 described, with tinted bands to reflect the radiations fall- 

 ing on it to another tablet iodized to a yellow color, and 

 placed in a camera. After due exposure and develop- 

 ment of both with mercury, it will be found that the im- 

 age of the first tablet formed on the second consists of 

 bands of different shades of whiteness. The yellow parts 

 of the first tablet have scarcely affected the second, but 

 its metallic and blue parts have acted very powerfully. 

 On comparing the first plate and its image on the second 

 together, it will be perceived that the parts that have 

 been affected on the one are less affected on the other. 



It may therefore be inferred that the yellow films are 

 sensitive because they absorb the incident radiation, and 

 the metallic and blue are insensitive because they re- 

 flect it. 



The effect, in whatever it may consist, which occurs 

 during the invisible modification is not durable : it grad- 

 ually passes away. If tablets that have received im- 

 pressions be kept for a time before developing, the im- 

 ages upon them gradually disappear. On these tablets 

 there is na lateral propagation of effect, nothing answer- 

 ing to conduction. 



On examining the operation of a radiation continuous- 

 ly applied to one of these sensitive films, it will be dis- 

 covered that a certain time elapses that is, a certain 

 amount of the radiation is consumed before there is any 

 perceptible effect. When that is accomplished, the radi- 

 ation affects the film to a degree proportional to its quan- 

 tity, until a second stage is reached ; there is then an- 

 other pause, followed by the second stage, in which vis- 



