of Bromide of Silver. 275 



powerful "sensitizer" (see the above work, 1873, p. 88); that is 

 to say, it augments their sensibility in consequence of its fixing 

 chemically the iodine or bromine as it becomes liberated during 

 exposure. That this action occurs chiefly in the blue finds its 

 solution without doubt in this, namely that the blue rays are 

 more eagerly absorbed by the wet film than are the others. 



As has been already observed, the sensibility of the dry bro- 

 mide of silver diminishes gradually from the blue to the red. 

 With bromide-of-silver plates as prepared by me I could see no- 

 thing of the phenomenon which I had noticed as occurring in 

 such a marked manner with the English bromide-of-silver plates 

 spoken of above, namely a falling off of the sensibility from the 

 violet to the blue, and an increase thereof from the blue to the 

 green. The explanation above offered as to the action of nitrate 

 of silver upon bromide of silver induced me, however, to con- 

 jecture that the English bromide-of-silver plates must contain 

 some substance that absorbs the green in a greater measure tlian 

 the blue. It is not unusual to give dry plates a coating of sub- 

 stances of the most varied kinds, such as gallic acid, caffeine, or 

 morphine, all which bodies fix iodine and bromine and exercise 

 a sensitizing action; occasionally, too, a yellowish colouring- 

 matter is added thereto, with the view of retarding the "chemical" 

 blue light thereby. The optical demeanour of these " preserva- 

 tives " may be looked upon as a matter not yet in any way un- 

 derstood ; neither, indeed, is their favourable influence placed 

 beyond all question. 



The plates of Wortley contain nitrate of uranium, gum, gallic 

 acid, and a yellow colouring- matter as a coating. In order to 

 ascertain whether this coating exercised any action, I washed one 

 of these plates with alcohol and water, and obtained in fact by 

 so doing a plate that no longer gave any indication of an aug- 

 mented sensibility in the green. I now made an attempt to 

 impregnate bromide of silver with a substance that absorbs espe- 

 cially the yellow rays and fixes free iodine or bromine, in the 

 hope of thereby heightening the sensibility for yellow. I selected 

 coralline, which Professor Liebermann most kindly placed at my 

 disposal. A solution thereof, when greatly diluted, gives in the 

 spectroscope an absorption-band between D and E ; in a more 

 concentrated solution the absorption becomes widened out until 

 it reaches beyond D, while, on the other hand, the blue near F is 

 transmitted to a pretty considerable amount. 



I dissolved coralline in alcohol and added some of it to my 

 bromide collodion, so that it became of a strong red colour. 

 Dry bromide of silver plates prepared with this collodion were 

 of a decided red tone. On exposure to the spectrum they bore 

 out my anticipation ; that is to say, the plates proved sensitive 



T2 



