208 Dr. Draper on Some Analogies between 



to the relative sensitiveness of the chlorides, bromides, oxides'* 

 and iodides of silver, &c. But it seems this liability to change 

 depends also on other principles, which, being liable to varia- 

 tion, the sensitiveness of a given body varies with them. Thus 

 this very iodide of silver, when in a thin yellow film, is de- 

 composed by the feeblest rays of a taper, and even moon- 

 light acts with energy; yet simply by altering the thickness 

 of its film it becomes sluggish, blackening even in the sun- 

 light tardily, and recovers its sensitiveness again on recover- 

 ing its yellow hue. 



(62.) We have now no difficulty in understanding, how in 

 the preparation of ordinary sensitive paper great variations 

 ensue, by modifying the process slightly, and how even on 

 a sheet which is apparently washed uniformly over, large 

 blotches appear which are either inordinately sensitive, or not 

 sensitive at all. If, without altering the chemical composition 

 of a film on metallic silver, or even its mode of aggregation, 

 such striking changes result by difference of thickness, how 

 much more may we expect that the great changes in molecu- 

 lar condition, which apparently trivial causes must bring 

 about on sensitive paper, should elevate or depress its capa- 

 bility of being acted on by light ! If I mistake not, it is upon 

 these principles that an explanation is to be given of the 

 successful modes of preparation which Mr. Talbot and Mr. 

 Hunt have described, and the action of the mordants of Sir 

 John Herschel. 



(63.) I therefore infer, 



6th. That the sensitiveness of any givenpreparationdepends 

 on its chemical nature, and its optical qualities conjointly ; and 

 that it is possible to exalt or diminish the sensitiveness of a 

 given compound, by changing its optical relations. 



(64-.) 7th. That, as when radiant heat falls on the surface of 

 an opake body, the number of rays refected is the complement 

 of those that are absorbed, so in the case of a sensitive prepara- 

 tion, the number of chemical rays refected from the surface is 

 the complement of those that are absorbed. 



This important proposition I prove in the following way : — 

 I take a plate, A G, fig. 4, three inches by four, and by 

 partially screening its surface whilst in the act of iodizing, 

 with a proper piece of flat glass, I produce upon it five trans- 

 verse bands, b, c, d, e,f; the fifth, f, which has been longest 

 exposed, is of a pale lavender colour ; the fourth a bright 

 blue ; the third a red ; the second a golden yellow ; and the 

 first uniodized metal ; the object of this arrangement being 

 to expose at the same time anil on the same plate, a series of 

 films of different colours and of different thickness, and to 



