432 Dr. J. W. Draper on the Distribution of 



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 becomes obvious when we cause a tablet, prepared 

 as just described, with tinted bands to reflect the radiations 

 falling on it to another tablet iodized to a yellow colour and 

 placed in a camera. After due exposure and development of 

 both with mercury, it will be found that the image 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 unaffected 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 reflect it. 



The effect, in whatever it may consist, which occurs during 

 the invisible modification is not durable; it gradually passes 

 away. If tablets that have received impressions be kept for a 

 time before developing, the images upon them gradually dis- 

 appear. 



On these tablets there is no lateral propagation of effect, 

 nothing answering to conduction. 



On examining the operation of a radiation continuously 

 applied to one of these sensitive films, it will be discovered 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 radiation affects the film to a degree propor- 

 tional to its quantity, until a second stage is reached. There 

 is then another pause, followed by the second stage, in which 

 visible modification or chemical decomposition sets in. The film 

 begins to darken ; it passes through successive tints, brown, red, 

 olive, blue, and eventually becomes dark grey. 



I have described in some of the foregoing paragraphs the 

 action of the spectrum on silver iodide, as presented on the 

 tablet of the Daguerreotype, showing the difference in the im- 

 pressions obtained, 1st, when extraneous light has been exclu- 

 ded j 2nd, when it has been permitted simultaneously or pre- 

 viously to act. 



In the latter case, in all that region of the spectrum from the 

 more refrangible extremity to somewhat beneath the line G, 

 the usual darkening effect, manifested by silver compounds, is 

 observed ; but below this, and to the extreme less refrangible 

 rays, with certain variations of intensity, the action of the ex- 



