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XXXII. On the Sensibility to Light of Bromide of Silver with 

 respect to the so-called Chemically Inert Colours. By Her- 

 mann Vogel*. 



Y\^ITH regard to certain colours (red, yellow, and green) the 

 * * photographic action, as we are aware, is either very limited 

 or altogether wanting. This circumstance not only throws dif- 

 ficulties in the way of copying coloured objects (oil paintings), 

 but also with regard to taking portraits, in which coloured 

 clothes and likewise a yellow tint of the complexion, light hair, 

 or rosy cheeks are reproduced in abnormal conditions. What is 

 of a light colour, when of a yellowish tint, comes out bright ; and 

 this drawback is only to be got over to a certain extent by a sub- 

 sequent retouching of the negative. 



This abnormal insensibility to colour, as regards photographic 

 plates, is pronounced in the most marked manner with respect 

 to the colours of the spectrum, wherein beyond the violet a 

 powerful action is manifested, which in the visible spectrum 

 (according to the researches hitherto made) does not extend 

 deeper than to the line E in the green. (See Dr. Schultz-Sellack, 

 ' Reports of the German Chemical Society/ 1871, p. 211). Re- 

 searches recently instituted by me by means of bromide of silver, 

 however, have shown that the sensibility of this preparation not 

 only embraces a considerably wider extent of the spectrum, but 

 even that, by the employment of certain accessories, its sensibi- 

 lity may be carried as far as into the red, or, in other words, to 

 regions where hitherto for photography dark night reigned. 



I received recently from England some dry bromide-of-silver 

 plates which Wortley prepares in that country for sale, by a pro- 

 cess which is only in part divulged. On exposing these plates 

 to the spectrum, I found to my astonishment that they proved 

 to be more sensitive in the green (that is to say, near the line E) 

 than in the light blue (that is to say, near the line F). Here 

 was an instance of sensibility not in accordance with our previous 

 experience — namely, that a colour the chemical action of which 

 was held to be weak, proved more energetic than that of one 

 which was looked upon as powerful. This circumstance induced 

 me to enter at once upon a more intimate examination of the 

 behaviour of bromide of silver with regard to the colours of the 

 spectrum. 



I formed my spectrum by the use of a photographic camera 

 and a Steinheil lens, which I adapted to the battery of prisms 

 of a direct-vision spectroscope. The width of the slit was 0*25 



* From Poggendorff 's Annalen, vol. cl. p. 453. Kindly communicated 

 by W. G. Lettsom. 



Phil. Mag. S. 4. Vol. 47. No. 312. April 1874. T 



