'200 Captain Abney on the Reversal of 



collimating lens being a duplicate of it. The time of exposure 

 was, as a rule, three minutes to the sunlight or to that of the 

 electric arc, care being taken in the latter case that an image 

 of the positive pole fell on the slit so as to give a continuous 

 spectrum. The action of potassium iodide on silver iodide will 

 first be described. 



A plate was exposed after being sensitized, and after wash- 

 ing was immersed in a cell containing a 1-per-cent. solution 

 of potassium iodide and exposed to the spectrum. The result 

 is shown in Plate V. fig. 1 ; the same rays which cause an 

 image to be formed in the nsual manner likewise caused a re- 

 versal (dotted curve, fig. 1). 



A plate similarly prepared was exposed in a 1-per-cent. 

 solution of potassium bromide for the same length of time, 

 with the result that a reversal was obtained in the blue and 

 likewise in the red, but much less marked in the latter (fig. 2). 

 These two experiments tend to prove that, in reality, it is the 

 bromide that is acted upon to some extent, and the effect is not 

 entirely due to the silver-salt. This was particularly manifest 

 in the case of the iodide and bromide slightly acidified with a 

 mineral acid, and was much less marked when the solution 

 was alkaline — in the latter case, the reversal taking place in 

 the blue, and not in the red regions of the spectrum. 



To see if the silver-salt had any marked effect on the rapidity 

 of oxidation, a silver-iodide plate was washed, given the same 

 preliminary exposure, and then placed in the spectro-photo- 

 grapic apparatus without any surrounding fluid. A reversal 

 was obtained in the blue, but not to any thing like such an ex- 

 tent as when placed in soluble iodides or bromides. The rever- 

 sal, therefore, when the plate is exposed in the latter is par- 

 tially due to the action of radiation on the bromide, and partly 

 to that exerted on the silver-salt itself. 



A silver-iodide plate, treated as before, was next exposed in 

 a weak solution of potassium bichromate, when there was a 

 strong reversal in the red (fig. 3), and no action whatever in 

 the blue. Permanganate of potash was next substituted for 

 the bichromate ; and the same reversing action was found, 

 with the addition of a negative image in the blue (fig. 4). 



With hydroxyl the same phenomena were observed as with 

 the permanganate, the reversal taking place a little further 

 into the green (fig. 5). Studying the absorption due to these 

 three oxidizing agents, it would appear that the reversing 

 action is clue to the action of light on the salt of silver, which is 

 changed by the preliminary exposure to light, and not to the action 

 of light on the medium in which the plates are placed. 



With mineral acids a reversal was always obtained in the 



