W. Crookes on the Wax-paper Photographic Process. 169 



One would think that in no part of the photographic operation, 

 would greater unanimity exist, than on the composition of the 

 iodizing bath ; but on this subject, strangely enough, no two 

 persons seem to think alike. The formulae for this bath are 

 nearly as numerous as the operators themselves, and some of 

 them show not a little ingenuity in the manner in which sub- 

 stances apparently the most unphotographic have been pressed 

 into service. 



33. The results of numerous experiments, which I need not 

 mention here, had convinced me, that for ordinary purposes, 

 iodid of silverier se was the best sensitive surface for receiving 

 an image in the Camera ; but on making use of that body in 

 these operations, (by employing pure iodid of potassium in the 

 bath,) I was surprised to meet with results, for which I was at 

 first unable to account. A little consideration, however, showed 

 me the direction in which I was to look for a remedy. The ex- 

 periments which had led me to prefer iodid of silver as a sensi- 

 tive surface, had all been performed with sunlight, either direct, 

 or more frequently in the form of diffused daylight. In this 

 case, however, coal gas was the source of light ; and if, as was 

 very probable, there were any great difference in the quality of 

 the light from these two sources, the superiority of iodid over 

 the bromid or chlorid of silver would still be a matter for ex- 

 periment. 



34. A comparison of the spectra of the two kinds of light 

 showed a very marked difference ; while in sunlight the spectral 

 rays which are around and above the fixed line Gr, (the indigo 

 and higher rays) are so intense and numerous, as completely to 

 overpower the small space between and about F and (t, (the blue 

 and upper portion of the green,) a part of the spectrum which 

 affects bromid more than iodid of silver ; in gaslight, the case 

 was quite different. The great bulk of photographic rays was 

 found to lie within the limits of the visible spectrum, and conse- 

 quently the photographic action of this light was likely to be far 

 more energetic on bromid than on iodid of silver. These suppo- 

 sitions were fully borne out by experiment : on introducing a 

 little bromid of potassium into the iodizing bath, the change was 

 very apparent. It requires a certain proportion to be observed 

 between the two to obtain the best results. If the iodid of 

 potassium be in excess, the resulting silver salt will be wanting 

 in sensitiveness, requiring a comparatively long development to 

 render an image visible; while, if the bromid be in excess, there 

 will be a great want of vigor in the impression, the picture being 

 red and transparent. When the proportion between the two is 

 properly adjusted, the paper will be extremely sensitive, the 

 picture presenting a vigorous black appearance, without the 

 least approach to red. The addition of a chlorid was found to 



SECOND SERIES, VOL. XXII. NO. 65. — SEPT., 1856. 



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