1886.] Different Parts of the Spectrum on Silver Salts. 



253 



that when the outside ground glass was illuminated by a candle about 

 3 feet away, the light shading every part of the bottom of each 

 chamber was for all practical purposes uniform. A patch of mono- 

 chromatic solar light from one part of the spectrum was then thrown 

 on the ground glass, and an exposure of 30 seconds given to a plate 

 in contact with the brass punctured plate at the bottom of the cham- 

 bers. Another portion of the spectrum was next thrown on a fresh 

 sensitive surface, and a similar operation carried out, and so on till 

 the whole of the range of the spectrum had been utilised. In each 

 set of experiments it is scarcely needful to remark the same batch of 

 plates was employed. All the plates were developed together for the 

 same length of time, and the number of the chamber noted where no 

 photographic action was visible. Thus if No. 8 showed a trace of 

 photographic action, and No. 9 showed none, No. 9 was taken as a 

 measure. All these numbers were then tabulated, and the admitted 

 light calculated. 



Another series of experiments were then conducted precisely as 

 before, the length of exposure being varied, and the numbers observed 

 were again tabulated and compared with the first set. A third series 

 was then taken, and a mean of the results taken. The plates were 

 next fixed and the numbers read, and the light again calculated, with 

 the result that the mean corresponded with the first mean. As a 

 final check, each set of plates were printed on uniformly sensitised 

 paper, and the gradations obtained by the method described in my 

 Treatise on Photography (Longmans). The results obtained were 

 almost identical with the first means. Various salts of silver and 

 combinations of salts were tried, but I need only give one, which is 

 that which has been disputed. The figure gives a graphic description 

 of the results obtained. This series of plates was prepared with a 

 mixture of 6 per cent, of iodide, and 94 per cent, of bromide of silver, 

 and the two were precipitated together. It was somewhat difficult 

 in a photograph of the spectrum, containing but little iodide, to be 

 sure of this dip at G, owing to the occurrence of Fraunhofer lines. 

 The method adopted brings the dip clearly into view. It might be 

 thought that the strong band of lines near G produced it, but sucli 

 is not the case, as pure bromide of silver without any admixture of 

 iodide did not show it, and the one maximum of sensitiveness it 

 had lay nearer G. 



In the mixed salt which was experimented upon we thus still get 

 two maxima, though the percentage of iodide and bromide is but 

 small. The same line of argument which was applied in the paper I 

 have already referred to as to the cause of this dip near G, still 

 therefore applies. 



