1881.] Spectrum on the Haloid Salts of Silver, fyc. 183 



iodide is not all destroyed, and we get the maximum corresponding* 

 with, the maximum of pure silver iodide, fig. 55. The same paper 

 developed with ferrous citrate shows a slight dip near G, fig. 56. 

 The difference in 55 and 56 is seemingly due to the fact that silver 

 iodide has more attractive power for precipitating metallic silver 

 than has the bromide (a fact which is well known) and that the 

 bromide is more amenable to reduction than is the iodide. 



Figs. 57 and 58 are well worthy of attention. They are the results 

 of the exposure of the same plate for different lengths of time to the 

 spectrum. It was prepared in the silver nitrate bath and exposed in 

 the presence of free silver nitrate. Taking fig. 57 alone, it might be 

 supposed that we had a similar case to that which we have recently 

 considered, since we find an extraordinarily (apparent) greater sensi- 

 tiveness in the green than in the violet, and yet we have the image 

 formed in the presence of an excess of silver nitrate, which would be 

 against the theory I have promulgated. Fig. 58, however, clears up 

 the discrepancy : the maximum is found to be at G, and in this case 

 the dip in the curve of fig. 57 is caused by the reversing action 

 alluded to. 



Fig. 59 gives the curve obtained by the above mixture of three 

 parts of iodide to one of bromide ; when emulsified in gelatine the 

 bottom curve shows a short exposure. 



One Part of Iodide and Three of Bromide. — We now come to a mix- 

 ture of one part of silver iodide to three of bromide. I have not 

 described the printed spectra since they correspond nearly with figs. 

 • 47 and 48. 



If we compare the curves in figs. 60 and 57 we see a strange simi- 

 larity between them, bat if we take into consideration fig. 61, which 

 is that due to a short exposure on the same plate, we shall at once see 

 that the dips about G are due to two different causes ; the dip in 

 fig. 61 is caused by the formation of the new molecule. Figs. 60 and 

 61 are also the curves shown by paper prepared with the above equi- 

 valents of iodide and bromide, and also of the same in collodion when 

 developed by an organic ferrous salt. When developed by acid deve- 

 lopment, the curve in the more refrangible region is a little more 

 pronounced in character. 



Figs. 62 and 63 show the same equivalents emulsified in gelatine 

 and developed by ferrous oxalate. 



The different equivalent proportions of bromide to iodide, it will be 

 noticed, show themselves in the curves more particularly when a com- 

 parison is made between figs. 51, 59, and 62. 



Mixture of Iodide and Chloride. 



Three Equivalents of Iodide to One of Chloride. — When paper 

 is prepared with three equivalents of silver iodide to one of 

 VOL. xxxm. o 



