168 



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



converting it into silver chloride. Exposure for five minutes to the 

 spectrum gave the result shown in fig. 5, in which it will be seen that 

 whilst the most refrangible portion took a grey colour, the small portion 

 below G became a pink, the line of demarcation between the two 

 being well defined. It now seemed probable that the pink part of the 

 spectrum was due to the chloride and the grey to the iodide. 



To further investigate the matter, the same paper without iodide 

 was floated on silver nitrate and exposed to the spectrum, with the 

 result given in fig. 6, a very faint trace of action being visible where 

 the paper was exposed for a quarter of an hour to the spectrum trans- 

 mitted by the potassium chromate. 



Iodised paper prepared as in the first experiment was well washed 

 and simply exposed with the result to be seen in fig. 7. Finally, paper 

 was prepared and washed, then immersed in a weak solution of 

 potassium iodide, washed well and flooded with potassium nitrite, and 

 the result is given in fig. 8. ISTow, fig. 1 coincides with the observa- 

 tions made by Sir J. Herschel, on paper similarly prepared, in 1842, 

 and described in the " Phil. Trans." for 1843, and he classes this 

 spectrum as due to the silver iodide. It will be seen that the printed 

 spectrum due to silver iodide is that given in fig. 8, and that the tail 

 extending to the least refrangible end is really due to the action of 

 that region on the organic salt (and perhaps chloride) of silver 

 present in the paper. Further, it will be seen that the greater part of 

 the darkening in fig. 1 of that tail is due to the action of the different 

 rays after or whilst diffused light has acted or is acting on that 

 organic compound. Confirmatory experiments were made with pure 

 silver iodide in collodion with excess of silver nitrate, and also without 

 such excess, with the result shown in fig. 8. 



If further confirmation were required, it was only necessary to add 

 to a film of collodion containing the iodide and excess of silver nitrate 

 a small trace of organic matter, such as resin or albumen, and the 

 result given in fig. 9 was obtained. 



Thus, then, we may say that the parts of the spectrum capable of 

 direct action on silver iodide are shown in fig. 8. 



The next point to which my attention was turned was to ascertain 

 the true region of the spectrum which was active on silver iodide 

 when developed. 



There are several developers for silver haloids : — 



f 1st. Ferrous sulphate and silver nitrate. 

 Acid developers . . < 2nd. Pyrogallic acid ,, ,, 



1 3rd. Gallic acid „ 

 Neutral organic iron f 4th. Ferrous oxalate. 



developers \ 5th. Ferrous citro-oxalate. 



Alkaline developers 6th. Pyrogallic acid and ammonia. 



