424 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



trum, as in the daguerreotype. The effect is altogether 

 due to the change of composition of the sensitive film. 

 Ordinarily it contains free nitrate; now it contains free 

 iodide, chloride, etc. 



The silver compounds of collodion absorb the radia- 

 tions falling on them which are capable of producing a 

 photographic effect. Yet, sensitive as it is, collodion is 

 very far from having its maximum sensitiveness, as is 

 shown by the following experiment, which is of no small 

 interest to photographers : I took five dry collodion 

 plates, prepared by what is known as the tannin process, 

 and, having made a pile of them, caused the rays of a gas- 

 flame to pass through them all at the same time. On 

 developing, it was found that the first plate was strongly 

 impressed, and the second, which had been behind it, ap- 

 parently quite as much. Even the fifth was considerably 

 stained. From this it follows that the collodion film, as 

 ordinarily used, absorbs only a fractional part of the 

 rays that can affect it. Could it be made to absorb the 

 whole, its sensitiveness would be correspondingly in- 

 creased. 



Radiations that have suffered complete absorption can 

 bring about no further change. Partial absorption, aris- 

 ing from inadequate thickness, may leave a ray possessed 

 of a portion of its power. There must be a correspond- 

 ence between the intensity of the incident ray and the 

 thickness of the absorbing medium to produce a maxi- 

 mum effect. 



Though the silver iodide is affected by radiations of 

 every refrangibility, it is decomposed so that a subiodide 

 results only by those of which the wave-length is less 

 than 5000. If in presence of metallic silver, as on the 

 daguerreotype tablet, the iodine disengaged unites with 

 the free silver beneath. The rays of high refrangibility 

 occasion in it chemical decomposition ; those of less re- 



