ON SELF-RECORDING MAGNETOGRAPKS. 213 



apparent until an after stage, while the saving of time and trouble is a great 

 temptation to beginners. It is to a neglect of this point that I am inclined 

 to attribute most of the faults so commonly laid to the charge of this beau- 

 tiful process ; such as gravelly appearance, or want of smoothness in the 

 lights, and quick decomposition in the developing solution. 



31. A well-waxed sheet of paper, when viewed by obliquely reflected 

 light, ought to present a perfectly uniform glazed appearance on one side, 

 while the other should be rather duller; there must be no shining patches 

 on any part of the surface, nor should any irregularities be observed on ex- 

 amining the paper with a black ground placed behind; seen by transmitted 

 light, it will appear opalescent, but there should be no approach to a granular 

 structure. The colour of a pile of waxed sheets is slightly bluish. 



32. The paper, having undergone this preparatory operation, is ready for 

 iodizing ; this is effected by completely immersing it in an aqueous solution 

 of an alkaline iodide, either pure or mixed with some analogous salt. 



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 substances 

 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, iodide of silver per 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 iodide 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 experiments which had 

 led me to prefer iodide of silver as a sensitive surface, had all been performed 

 with sunlight, either direct, or more frequently in the form of diffused day- 

 light. 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 iodide over the bromide or chlo- 

 ride of silver would still be a matter for experiment. 



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 G (the indigo and higher rays) are so intense and 

 numerous, as completely to overpower the small space between and about 

 F and G (the blue and upper portion of the green), a part of the spectrum 

 which affects bromide more than iodide 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 consequently the photographic action 

 of this light was likely to be far more energetic on bromide than on iodide 

 of silver. These, suppositions were fully borne out by experiment : on intro- 

 ducing a little bromide 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 iodide of potassium be in excess, the 

 resulting silver salt will be wanting in sensitiveness, requiring a compa- 

 ratively long development to render an image visible ; while, if the bromide 

 be in excess, there will be a great want of vigour 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 pre- 

 senting a vigorous black appearance, without the least approach to red. The 

 addition of a chloride was found to produce a somewhat similar effect to that 



