1881.] Spectrum on the Haloid Salts of Silver, Sec. 167 



Localities of Maximum Action. — I have followed the usual custom 

 of writers ou this subject, aud showu the top of my curves as the 

 place of maximum action. Although this correctly shows what 

 appears on the photographic plate, yet in all cases it is apt to give a 

 false notion regarding the effect of the spectrum. If we look at the 

 energy of the spectrum in its different localities, we find that it 

 rapidly decreases as it approaches the violet and ultra-violet. If this 

 diminution of energy be taken into account, it will be found that 

 usually the point of maximum effect nearest the violet indicates the 

 region where the absorption of the rays becomes total, and that the 

 shading off towards the ultra-violet is really only due to the diminished 

 energy of that part of the spectrum. In other cases, as, for instance, 

 where there are two maxima, this will not apply to the second 

 maximum. 



Silver Iodide. 



Visible Effect of the Spectrum on Silver Iodide. — If paper be soaked 

 in a 10 per cent, solution of potassium iodide and dried, and then be 

 floated on a 10 per cent, solution of silver nitrate and exposed whilst 

 moist,* the spectrum will be impressed in five minutes, as given in 

 fig. 1, where it will be seen that the whole visible spectrum is 

 impressed. Similar paper if exposed to the spectrum coming through t 

 a weak solution of potassium chromate, exhibits after ten minutes a 

 slight action in the least refrangible region (fig. 3) . If, however, the 

 paper be exposed for ten seconds to diffused light and then be exposed 

 to the same spectrum as the last the action is more intense than before, 

 though the exposure be for only two minutes (fig. 2). From this we 

 learn that part of the action of the spectrum in fig. 1 is due to the 

 action of diffused light. It next remained to trace the action on the 

 different silver compounds existing in this paper, which was ordinary 

 sized saxe paper. Paper was prepared as before, but washed in common 

 water till nearly all excess of silver nitrate was eliminated, and it was 

 then given a wash of potassium nitrite, an absorbent of iodine. Such 

 paper was exposed to the spectrum, first, coming through chromate, 

 second, unshaded. The print obtained is that shown in fig. 4, by 

 which it will be seen that the same limits were reached as before, but 

 that there is not that abrupt descent of sensitiveness near Gr; 

 evidently some cause of the extreme sensitiveness near this point had 

 been eliminated, and apparently that could only be the silver nitrate 

 and the presence of the potassium nitrite. To test the matter 

 further, paper was prepared in the same manner, but before applying 

 the potassium nitrite it was soaked in common salt and water and 

 washed. This would effectually remove all traces of silver nitrate 



* The same action was observed where the paper was allowed to dry, but the 

 darkening was less. 



VOL. XXXIII. N 



