170 



Capt. Abney. On the Effect of the [Dec. 15, 



Now, the first three gave precisely similar results as did the last 

 three. It will, therefore, be unnecessary for me to state for every ex- 

 periment which developer was used. With collodion or gelatine plates 

 I preferred the 2nd and the 4th developers, and with paper the 3rd 

 and the 5th. 



It may be necessary to point to the different materials employed. 

 In the first place very pure potassium iodide was obtained by Stas's 

 method, and as much as would dissolve was put into collodion ; by 

 the free use of water with the alcohol as much as 4 grs. was dissolved. 

 This was employed with a silver bath prepared in the usual way, con- 

 taining 35 grs. of silver nitrate to each ounce of water. 



5 grs. of commercial cadmium iodide was dissolved in an ounce of 

 collodion, and this was also used with a silver nitrate sensitising bath. 

 The pyroxylin forming the collodion was carefully selected. Before 

 taking into use it had been precipitated from solution by water, 

 washed in alcohol, again precipitated, and washed and dried, and then 

 redissolved in equal parts of pure ether and alcohol at the rate of 

 7 grs. to each ounce. Such a solution after prolonged exposure when 

 impregnated with nitrate of silver gave no reduction of the salt. 



The emulsions of silver iodide were made by dissolving 6 grs. 

 of silver nitrate in alcohol, adding this to collodion, and gently adding 

 the equivalent to 5 grs. of silver nitrate of the soluble iodide (dis- 

 solved in alcohol) to it. This formed a perfect emulsion of silver 

 iodide in the presence of a slight excess of silver nitrate, and also of 

 course of the soluble nitrates formed by the double decomposition of 

 the above. I may at once say that the presence or absence of these 

 soluble nitrates had no effect at all on the results, and may at once be 

 dismissed from further consideration. 



Gelatine emulsion was prepared in the same manner, keeping in 

 mind, however, that in this case it was prepared with an excess of 

 soluble iodide instead of silver nitrate. It is well to remark that it is 

 impossible to get a fine emulsion of silver iodide in collodion unless 

 the plan indicated above be followed of first dissolving the silver 

 nitrate in the collodion and then adding the iodide to that, in addition 

 to which it is necessary that the silver nitrate be in excess or the 

 emulsion becomes granular. With gelatine the emulsification is an 

 easier matter, but in order to prevent spontaneous decomposition of 

 the gelatine it is necessary that the soluble iodide be in excess. 

 Emulsions of both kinds were " washed " by the usual methods known 

 to photographers. In the case of the collodio-iodide of silver great 

 care was taken that nothing but pure distilled water was employed. 



It will be well to show here how it was we ascertained that nothing 

 but pure iodide of silver exists in a film. The impurities to be met 

 with are oxides, chlorides, and bromides. Now when an oxide of 

 silver, or silver chloride or bromide is placed in a solution of potas- 



