172 



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



nitrite, when developed by the acid or other methods, gave the re- 

 sults in figs. 10 and 11. The purification of silver iodide by this treat- 

 ment cut off the small tail on the least refrangible side of G seen in 

 fig. 14. When the pure silver iodide prepared by the aid of the pure 

 potassium iodide was used, figs. 13 and 12 resulted. A plate was next 

 coated with collodion iodised with the pure potassium iodide, im- 

 mersed in the bath, washed, and then placed in a solution of common 

 salt (1 gr. to 5 oz.), with the result that figures similar to figs. 16 and 

 17 were obtained. 



A plate similarly treated, except that potassium bromide was sub- 

 stituted for the common salt, gave as a result figs. 16 and 17. There 

 was no marked difference whether fche plate was developed by the acid 

 developer or by the ferrous oxalate. It would be useless to describe 

 the many other experiments which were made, all tending to prove 

 that the true action of the spectrum on silver iodide in collodion is 

 that given in figs. 10 and 11. No deviation from it has been obtained, 

 unless impurity in the pyroxyline or in the soluble iodide was proved 

 to exist. 



With gelatine emulsions of yellow silver iodide, when rendered 

 sensitive by the use of potassium nitrite or silver nitrate, the same 

 action was found to hold good, and the same may be said for plates 

 prepared with albumen as a vehicle, when all the silver was converted 

 into iodide, and the sensitising was effected by potassium nitrite or 

 some other similar sensitiser. 



We next come to the iodide of silver when held in situ by paper. 

 The same method of preparation was adopted as that given above for 

 the printing experiments. When paper was exposed with the excess 

 of silver nitrate, on acid development fig. 18 was obtained. When 

 developed by an organic ferrous developer, fig. 19 was obtained ; 

 figs. 14 and 15 were obtained when similar paper was washed and 

 salted with common salt, and washed again, and then sensitised with 

 potassium nitrite. 



Figs. 18 and 19 are worthy of attention. It is seen in fig. 18 that 

 the iodide has much greater power of attracting freshly deposited 

 silver than have the impurities present with it in the paper. On the 

 other hand, fig. 19 shows that the ferrous oxalate developer has more 

 power of reducing the impurity (or rather the reduction is better seen) 

 than it has the iodide. 



When silver iodide paper is prepared and washed, and treated with 

 a weak solution of potassium iodide and resensitised by potassium 

 nitrite, figs. 10 and 11 are obtained. 



Fig. 20 shows the action of the spectrum on pure iodide when the 

 exposure is very prolonged. It appears as if the sensitiveness on the 

 more refrangible side of G had diminished. This is not the case, 

 however. The prolonged exposure causes a commencement of what is 



