1881.] Spectrum on the Haloid Salts of Silver, fyc. 173 



called a reversal of the image due to oxidation, which I have already 

 investigated in the " Philosophical Magazine," 1880, and the maximum 

 effect has, therefore, apparently shifted to the least refrangible side of 

 G, as shown. This is important, since phenomena which have been 

 described and figured by other investigators can be shown to be caused 

 by this reversing action. I shall have to allude to it myself again 

 further on. 



What has been noted regarding the action of impurities in the silver 

 iodide points to a method of ascertaining if an iodide or iodine itself 

 is pure. It is believed that the merest trace of impurity may be 

 recognised by this method of spectrum analysis. 



Silver Bromide. 



When paper is immersed in a 10 per cent, solution of potassium 

 bromide, then dried and floated on a 10 per cent, solution of silver 

 nitrate, and exposed to the action of the spectrum, the visible effect 

 will be observed as shown in fig. 21. Figs. 22 and 23 show the action 

 of the spectrum after filtration through potassium chromate, the 

 former being what is observed after a preliminary exposure to diffused 

 white light, and the latter when the paper has only seen the yellow 

 light. It is needless to go into all the details which were described 

 when silver iodide paper was under examination. The same causes 

 exist for the shape of the curve as they do with the latter paper. It 

 may be interesting to remark that the spectrum observed on paper 

 which has been washed and treated with potassium bromide after sen- 

 sitising is the same as that shown in fig. 25, whilst when only washed 

 and not treated with the soluble bromide it takes the form of fig. 29. 

 The reason of these differences in shape of curve is apparent when it 

 is remembered how the effects on silver iodide paper were traced to 

 their source. 



It must be noted that there are several molecular modifications of 

 .silver bromide. The first is that form in which it exists in the paper 

 and also in collodio-bromide emulsions when prepared in the ordinary 

 way; also when prepared in collodion by the bath. This form 

 transmits a yellow- orange tint when white light traverses it. Another 

 form is one which I described in the Bakerian Lecture for 1880, viz. 

 a form which transmits a blue-green tint ; and a last form which 

 transmits a grey tint, which is found in gelatine emulsions which 

 have been boiled, or treated with ammonia in the manner which is 

 •common at the present day. These three varieties were examined both 

 for the visible action of light and also for development. A plate was 

 -coated with the first emulsion named, with the result that the direct 

 action of light gave fig. 25. The blue-green transmitting form gave 

 fig. 24. This form is one which is sensitive to the infra-red rays of 

 the spectrum on development, and it will be seen that the printing 



