MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 423 



reversing actions cannot be obtained by the less refran- 

 gible rays. The spectrum impression developed in the 

 usual manner by an iron salt presents a sudden maxi- 

 mum about the line G, and continues thence to the high- 

 est limit of the spectrum. In the other direction it ex- 

 tends below F. From E to the ultra-red not a trace of 

 action can be detected. The lines o,j3, 7 cannot be ob- 

 tained on collodion. There is, therefore, a difference be- 

 tween its behavior under exposure to light and that of 

 a daguerreotype tablet. 



The reversals that are obtained on collodion by the 

 use of certain 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 dip- 

 ping the plate into a weak solution of iodide of potas- 

 sium, exposing it to the spectrum, washing, again dip- 

 ping it into the nitrate bath, and finally developing, a re- 

 versed action is obtained. The daylight is perfectly neu- 

 tralized, but not after the manner in a daguerreotype. 

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

 collodion, the impression of the light is totally removed 

 by an exposure 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 refrangible rays show no action. 



The results are substantially the same when, instead 

 of iodide of potassium, the chloride of sodium, corrosive 

 sublimate, bromide 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 portion of the spec- 



