182 



Capt. Abney. On the Effect of the 



[Dec. 15, 



liberated from this new molecule I am not at present prepared to 

 state, but it is my belief that it is the iodine, since density in develop- 

 ment by the alkaline method is readily obtained when experimenting 

 with it. 



To sum np, the difference in shape between curves 47 and 48 seems 

 to depend on the destruction of the sub-iodide when formed, and its 

 conversion into a new molecule, which is sensitive to the blue-green, 

 the same new molecule being formed by the liberation of the bromine 

 from the molecule of silver bromide when the sub-bromide is formed. 

 In the case of the paper which is dried in the presence of silver nitrate 

 and potassium nitrite the same result occurs. Bromine and iodine 

 attack these salts when in a crystalline state with difficulty, and hence 

 will in preference form the new molecules as before. 



Fig. 49 shows the curve of washed paper when developed with 

 ferrous citro- oxalate, and nearly the same result is seen when the 

 development proceeds by acid development, the difference being that 

 the dip in the curve between h and Gr is less pronounced. To illustrate 

 this further, in Fig. 50 we have the case of a collodion film containing 

 equal parts of silver iodide and silver bromide and an excess of silver 

 nitrate, the latter salt absorbing both the iodine and bromine libe- 

 rated. In fig. 52 we have the results obtained from the same film, 

 but thoroughly washed from all excess of silver nitrate. Whether the 

 plates be developed by acid, alkaline, or an organic ferrous salt, the 

 curves remain in all essential particulars the same. In fig. 51 we have 

 the curve resulting from the same mixture (equal equivalent propor- 

 tions) held in gelatine when developed by ferrous oxalate or alkaline 

 developer. At first sight it might be said that this action is really 

 due to the "reversing action" of light, of which I have treated in 

 the "Proceedings of the Royal Society," in 1878, and in the " Philo- 

 sophical Magazine " for 1880. That this is not the case is shown by 

 fig. 53, in which the exposure was exceedingly short ; in fact, when 

 very quick exposure was given the curve started at h and reached a 

 maximum as shown in fig. 53. These results are exceedingly inte- 

 resting and important. There is a figure showing something some- 

 what similar in Vogel's " Lehrbuch der Photographie," Berlin, 1878, 

 •but there is no explanation of the cause, nor has it been noticed by any 

 other observer, as far as I am aware. 



Three Parts of Iodide to One of Bromide. — When we take three 

 equivalents of silver iodide to one of bromide the curves are somewhat 

 modified. When washed paper prepared with the above proportions 

 is allowed to print in the spectrum, we have the curve shown in fig. 54. 

 When exposed damp in the presence of silver nitrate or other inor- 

 ganic sensitiser, we have almost a facsimile of the curve in fig. 47. 



Washed paper developed with acid developer shows that the propor- 

 tion of iodide is so large in comparison to the bromide that the sub- 



