Chemical Force in the Spectrum. 435 



tween its behaviour under exposure to light, and that of a 

 Daguerreotype tablet. 



The reversals that are obtained on collodion by the use of 

 haloid compounds are altogether different from the reversals on 

 the thin films of a silver tablet. They are produced by the 

 more refrangible rays. 



On exposing a collodion surface prepared in the usual manner 

 to daylight long enough to stain it completely, then washing off 

 the free nitrate, and in succession dipping the plate into a weak 

 solution of iodide of potassium, exposing it to the spectrum, 

 washing, again dipping it into the nitrate bath, and finally 

 developing, a reverse action is obtained. The daylight is per- 

 fectly neutralized, but not after the manner of a Daguerreotype. 

 In the region about G, the place of maximum action in collo- 

 dion, the impression of the light is totally removed by an expo- 

 sure of five seconds. In twelve seconds the protected space is 

 much larger; in thirty seconds it has spread from F to H. It 

 is, however, to be particularly remarked that the less refran- 

 gible rays show no action. 



The results are substantially the same when, instead of io- 

 dide of potassium, chloride of sodium, corrosive sublimate, bro- 

 mide of potassium, or fluoride of potassium is used. In all 

 these the reversing action is from F to H, and has its maximum 

 somewhere about G ; that is, the reversing action coincides 

 with the direct action ; there is no protection in the lower por- 

 tion of the spectrum, 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, &c. 



The silver compounds in collodion absorb the radiations fall- 

 ing 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 experi- 

 ment, w^bich 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 im- 

 pressed, and the second (which had been behind it) apparently 

 quite as much; even the fifth was considerably stained. From 

 this it follows that the collodion film, as ordinarily used, absorbs 

 only a fractional portion of the rays that can affect it. Could it 

 be made to absorb the whole, its sensitiveness would be corre- 

 spondingly increased. 



A ray that has suffered complete absorption can bring about 

 no further change; partial absorption, arising from inadequate 



2F2 



